Initial patch of the new mirror-less build-system for gst-ffmpeg using specific revisions of ffmpeg svn instead.

Original commit message from CVS:
* Makefile.am:
* autogen.sh:
* configure.ac:
* ext/ffmpeg/Makefile.am:
* ffmpegrev:
* gst-libs/ext/Makefile.am:
Initial patch of the new mirror-less build-system for gst-ffmpeg using
specific revisions of ffmpeg svn instead.
Might still have some issues, we need people to try this.
Help by : Dejan Sakelšak  <sakdean at gmail dot com>
* ext/ffmpeg/gstffmpeg.c: (plugin_init):
* ext/ffmpeg/gstffmpegcodecmap.c: (gst_ffmpeg_codecid_to_caps),
(gst_ffmpeg_caps_with_codecid), (gst_ffmpeg_caps_to_codecid),
(gst_ffmpeg_get_codecid_longname):
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_base_init),
(gst_ffmpegdec_get_buffer), (gst_ffmpegdec_audio_frame),
(gst_ffmpegdec_register):
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_averror),
(gst_ffmpegdemux_register):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_register):
* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_register):
Update code for new ffmpeg API.