Q:

No resource found that matches the given name in library

When trying to build my project, either via Android studio or via Gradle, I get these error messages:

/home/geecko/Documents/Android/OpenLauncher/app/build/intermediates/manifests/debug/AndroidManifest.xml:95:
  error: Error: No resource found that matches the given name (at
  'theme' with value '@style/Theme. WallpaperCropper'). 
  /home/geecko/Documents/Android/OpenLauncher/app/build/intermediates/manifests/debug/AndroidManifest.xml:108:
  error: Error: No resource found that matches the given name (at 'icon'
  with value '@mipmap/ic_launcher_wallpaper'). 
  /home/geecko/Documents/Android/OpenLauncher/app/build/intermediates/manifests/debug/AndroidManifest.xml:108:
  error: Error: No resource found that matches the given name (at
  'label' with value '@string/crop_wallpaper'). 
  /home/geecko/Documents/Android/OpenLauncher/app/build/intermediates/manifests/debug/AndroidManifest.xml:108:
  error: Error: No resource found that matches the given name (at
  'theme' with value '@style/Theme. WallpaperCropper')

Those resources are referenced in my Manifest and declared in a library (WallpaperPicker, package "com.android.launcher3") which compiles perfectly (I can build it with gradle and even see the .aar file) and is listed as a dependency in the main app's build.gradle. 
Here are my files:
-> /settings.gradle
include ':app' 
include ':WallpaperPicker'

-> /app/build.gradle
    buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:0.12.+'
    }
}

    apply plugin: 'com.android.application'

    android {
    compileSdkVersion 20
    buildToolsVersion '20.0.0'
    defaultConfig {
        applicationId 'be.geecko.openlauncher'
        minSdkVersion 16
        targetSdkVersion 19
        versionCode 1
        versionName '0.1'
    }
    buildTypes {
        release {
            runProguard false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

    dependencies {
    compile project(':WallpaperPicker')
    compile fileTree(dir: 'libs', include: ['*.jar'])
}

-> /app/AndroidManifest.xml
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="be.geecko.openlauncher">

    <permission
        android:name="com.android.launcher3.permission. PRELOAD_WORKSPACE"
        android:permissionGroup="android.permission-group. SYSTEM_TOOLS"
        android:protectionLevel="signatureOrSystem" />
    <permission
        android:name="com.android.launcher.permission. INSTALL_SHORTCUT"
        android:permissionGroup="android.permission-group. SYSTEM_TOOLS"
        android:protectionLevel="dangerous"
        android:label="@string/permlab_install_shortcut"
        android:description="@string/permdesc_install_shortcut" />
    <permission
        android:name="com.android.launcher.permission. UNINSTALL_SHORTCUT"
        android:permissionGroup="android.permission-group. SYSTEM_TOOLS"
        android:protectionLevel="dangerous"
        android:label="@string/permlab_uninstall_shortcut"
        android:description="@string/permdesc_uninstall_shortcut" />
    <permission
        android:name="com.android.launcher3.permission. READ_SETTINGS"
        android:permissionGroup="android.permission-group. SYSTEM_TOOLS"
        android:protectionLevel="normal"
        android:label="@string/permlab_read_settings"
        android:description="@string/permdesc_read_settings" />
    <permission
        android:name="com.android.launcher3.permission. WRITE_SETTINGS"
        android:permissionGroup="android.permission-group. SYSTEM_TOOLS"
        android:protectionLevel="normal"
        android:label="@string/permlab_write_settings"
        android:description="@string/permdesc_write_settings" />

    <permission
        android:name="com.android.launcher3.permission. RECEIVE_LAUNCH_BROADCASTS"
        android:protectionLevel="signature" />

    <uses-permission android:name="android.permission. CALL_PHONE" />
    <uses-permission android:name="android.permission. SET_WALLPAPER" />
    <uses-permission android:name="android.permission. SET_WALLPAPER_HINTS" />
    <uses-permission android:name="android.permission. VIBRATE" />
    <uses-permission
        android:name="android.permission. BIND_APPWIDGET"
        tools:ignore="ProtectedPermissions" />
    <uses-permission android:name="android.permission. GET_ACCOUNTS" />
    <uses-permission android:name="android.permission. READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="com.android.launcher.permission. READ_SETTINGS" />
    <uses-permission android:name="com.android.launcher.permission. WRITE_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission. READ_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission. WRITE_SETTINGS" />
    <uses-permission android:name="com.android.launcher3.permission. RECEIVE_LAUNCH_BROADCASTS" />

    <application
        android:name="com.android.launcher3.LauncherApplication"
        android:label="@string/application_name"
        android:icon="@mipmap/ic_launcher_home"
        android:hardwareAccelerated="true"
        android:largeHeap="@bool/config_largeHeap"
        android:supportsRtl="true">

        <uses-library
              android:name="com.android.launcher3" 
              android:required="true"/>

        <activity
            android:name="com.android.launcher3.Launcher"
            android:launchMode="singleTask"
            android:clearTaskOnLaunch="true"
            android:stateNotNeeded="true"
            android:theme="@android:style/Theme"
            android:windowSoftInputMode="adjustPan"
            android:screenOrientation="nosensor">
            <intent-filter>
                <action android:name="android.intent.action. MAIN" />

                <category android:name="android.intent.category. HOME" />
                <category android:name="android.intent.category. DEFAULT" />
                <category android:name="android.intent.category. MONKEY" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.android.launcher3.ToggleWeightWatcher"
            android:label="@string/toggle_weight_watcher"
            android:enabled="@bool/debug_memory_enabled"
            android:icon="@mipmap/ic_launcher_home">
            <intent-filter>
                <action android:name="android.intent.action. MAIN" />

                <category android:name="android.intent.category. DEFAULT" />
                <category android:name="android.intent.category. LAUNCHER" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.android.launcher3.LauncherWallpaperPickerActivity"
            android:theme="@style/Theme. WallpaperCropper"
            android:label="@string/pick_wallpaper"
            android:icon="@com.android.launcher3:mipmap/ic_launcher_wallpaper"
            android:finishOnCloseSystemDialogs="true"
            android:process=":wallpaper_chooser">
            <intent-filter>
                <action android:name="android.intent.action. SET_WALLPAPER" />
                <category android:name="android.intent.category. DEFAULT" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.android.launcher3.WallpaperCropActivity"
            android:theme="@style/Theme. WallpaperCropper"
            android:label="@string/crop_wallpaper"
            android:icon="@mipmap/ic_launcher_wallpaper"
            android:finishOnCloseSystemDialogs="true"
            android:process=":wallpaper_chooser">
            <intent-filter>
                <action android:name="android.service.wallpaper. CROP_AND_SET_WALLPAPER" />
                <category android:name="android.intent.category. DEFAULT" />
                <data android:mimeType="image/*" />
            </intent-filter>
        </activity>

        <!--  Debugging tools -->
        <activity
            android:name="com.android.launcher3.MemoryDumpActivity"
            android:theme="@android:style/Theme. NoDisplay"
            android:label="@string/debug_memory_activity"
            android:enabled="@bool/debug_memory_enabled"
            android:excludeFromRecents="true"
            android:icon="@mipmap/ic_launcher_home">
            <intent-filter>
                <action android:name="android.intent.action. MAIN" />

                <category android:name="android.intent.category. DEFAULT" />
                <category android:name="android.intent.category. LAUNCHER" />
            </intent-filter>
        </activity>

        <service
            android:name="com.android.launcher3.MemoryTracker"
            android:enabled="@bool/debug_memory_enabled"></service>

        <!--  Intent received used to prepopulate the default workspace. -- >
        <receiver
            android:name="com.android.launcher3.PreloadReceiver"
            android:permission="com.android.launcher3.permission. PRELOAD_WORKSPACE">
            <intent-filter>
                <action android:name="com.android.launcher3.action. PRELOAD_WORKSPACE" />
            </intent-filter>
        </receiver>

        <receiver android:name="com.android.launcher3.WallpaperChangedReceiver">
            <intent-filter>
                <action android:name="android.intent.action. WALLPAPER_CHANGED" />
            </intent-filter>
        </receiver>

        <!--  Intent received used to install shortcuts from other applications -->
        <receiver
            android:name="com.android.launcher3.InstallShortcutReceiver"
            android:permission="com.android.launcher3.permission. INSTALL_SHORTCUT">
            <intent-filter>
                <action android:name="com.android.launcher3.action. INSTALL_SHORTCUT" />
            </intent-filter>
        </receiver>

        <!--  Intent received used to uninstall shortcuts from other applications -->
        <receiver
            android:name="com.android.launcher3.UninstallShortcutReceiver"
            android:permission="com.android.launcher3.permission. UNINSTALL_SHORTCUT">
            <intent-filter>
                <action android:name="com.android.launcher3.action. UNINSTALL_SHORTCUT" />
            </intent-filter>
        </receiver>

        <!--  New user initialization; set up initial wallpaper -->
        <receiver
            android:name="com.android.launcher3.UserInitializeReceiver"
            android:exported="false">
            <intent-filter>
                <action android:name="android.intent.action. USER_INITIALIZE" />
            </intent-filter>
        </receiver>

        <receiver android:name="com.android.launcher3.PackageChangedReceiver">
            <intent-filter>
                <action android:name="android.intent.action. PACKAGE_CHANGED" />
                <action android:name="android.intent.action. PACKAGE_REPLACED" />
                <action android:name="android.intent.action. PACKAGE_REMOVED" />

                <data android:scheme="package"></data>
            </intent-filter>
        </receiver>

        <!--  The settings provider contains Home's data, like the workspace favorites -->
        <provider
            android:name="com.android.launcher3.LauncherProvider"
            android:authorities="com.android.launcher3.settings"
            android:exported="true"
            android:writePermission="com.android.launcher3.permission. WRITE_SETTINGS"
            android:readPermission="com.android.launcher3.permission. READ_SETTINGS" />

        <meta-data
            android:name="android.nfc.disable_beam_default"
            android:value="true" />

    </application>
</manifest>

So it seems like my main project (':app') can't access the resources of my library (':WallPaperPicker').  I can't figure out why... Here's the content of the project root:

app/           gradle.properties  settings.gradle
  build/         gradlew              WallpaperPicker/
  build.gradle  gradlew.bat          local.properties
  gradle/        OpenLauncher.iml

Any idea?  I really can't figure it out..

A:

after guessing some stuff, i got the idea that the library wasn't declared as android-library.  So in this case editing the build.gradle to 
apply plugin: 'com.android.library'

should do the trick.