Backport BRANCH-THREADED to HEAD, fix a bit.

Backport BRANCH-THREADED to HEAD, fix a bit.

Original commit message from CVS:
* configure.ac:
* ext/ffmpeg/Makefile.am:
* ext/ffmpeg/gstffmpeg.c: (plugin_init):
* ext/ffmpeg/gstffmpegcodecmap.c: (gst_ffmpeg_caps_with_codecid):
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_class_init),
(gst_ffmpegdec_init), (gst_ffmpegdec_query), (gst_ffmpegdec_event),
(gst_ffmpegdec_open), (gst_ffmpegdec_setcaps),
(gst_ffmpegdec_get_buffer), (gst_ffmpegdec_negotiate),
(gst_ffmpegdec_frame), (gst_ffmpegdec_sink_event),
(gst_ffmpegdec_chain), (gst_ffmpegdec_change_state),
(gst_ffmpegdec_register):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_class_init),
(gst_ffmpegenc_init), (gst_ffmpegenc_getcaps),
(gst_ffmpegenc_setcaps), (gst_ffmpegenc_chain_video),
(gst_ffmpegenc_chain_audio):
* ext/libpostproc/Makefile.am:
Backport BRANCH-THREADED to HEAD, fix a bit.