gst-ffmpeg: Use internal libav instead of external ffmpeg

External ffmpeg 1.2.4 is incompatible, gst-ffmpeg suggests
to always use internal libav.

Adding one patch:
Fixes compiling with gcc-4.7: gst-ffmpeg-0.10.13_gcc47.patch (taken from Slackware)

Fixes
http://autobuild.buildroot.net/results/ab8/ab8ec4904020ceb848a1e50f1865f594f46b8295/
http://autobuild.buildroot.net/results/f0a/f0a420015a92ebb8868c41c0dbf82cf27b2bdd0f/



Signed-off-by: Bernd Kuhls <berndkuhls@hotmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>