Update build.rb to support legacy build settings

Build output directory is parsed from xcodebuild to get the desired products directory. This allows users with legacy project settings and user defaults to use the build script for custom builds.

$ defaults read com.apple.dt.Xcode IDEApplicationwideBuildSettings
{
    OBJROOT = "/Users/nick/Source/Builds/Intermediates";
    SYMROOT = "/Users/nick/Source/Builds";
}