XQuartz: Create Logs dir if it does not exist

Apparently Apple devs just rely on other apps to create the `~/Library/Logs` directory.

This error came up when installing XQuartz on a fresh OSX VM (with XCode CLI tools + Homebrew + Homebrew Cask):
![screenshot](https://cloud.githubusercontent.com/assets/287584/3278643/bb47f83c-f3cb-11e3-8ef9-7aabea211362.png)

I have many other apps using that directory on my "host" OSX, any of them theoretically could have the same problem, but at least one of these have managed to create the directory (I haven't created the directory manually).
```
$ ls ~/Library/Logs
ACC.log				MusicManager
AMRestoreLog.txt		NELog.log
Adobe				PDApp.log
Adobe Illustrator CS6		ScreenRecycler.log
AdobeDownload			SparkleUpdateLog.log
AdobeIPCBroker.log		Spotify
AdobeIPCBrokerCustomHook.log	Ubiquity
AndroidStudioPreview		Unity
CleanMyMac 2.log		VMware
DiagnosticReports		VMware Fusion
DiscRecording.log		WebIde50
DiskUtility.log			X11
FaceTime			appstore.log
FlashPlayerInstallManager.log	com.macpaw.CleanMyMac2
Garmin				fsck_hfs.log
GoogleSoftwareUpdateAgent.log	iOS Simulator
Homebrew			iStatMenus.log
ICDM				iStatMenus2.log
Last.fm				pip.log
Last.fm Twiddly.log		storeagent
Last.fm iTunes Plugin.log	talagent.log
Last.fm.log			vnc
MTPViewer
```