Bug 851290 - Use GStreamer on Mac for H.264/MP3/AAC playback

Add support for in-tree gstreamer to the build system. Add
--with-system-gstreamer to build against the system wide installation of
gstreamer.
Bug 851290 - Use GStreamer on Mac for H.264/MP3/AAC playback

Add moz.build files
Bug 851290 - Use GStreamer on Mac for H.264/MP3/AAC playback

Import a subset of GStreamer 1.2.2. The files from gst-plugins-bad are a
snapshot from git (including VideoToolbox enhancements).