Original commit message from CVS:
* Makefile.am:
* configure.ac:
* ext/ffmpeg/Makefile.am:
* ext/ffmpeg/gstffmpeg.c:
* ext/ffmpeg/gstffmpegdemux.c:
* ext/libpostproc/Makefile.am:
* ext/libpostproc/gstpostproc.c:
Allow building against an external FFMpeg install. Fixes: #363363
Patch by: Josselin Mouette <joss at debian dot org>
When built against an external install, the plugin description will
say "system install" instead of "local snapshot"