configure.ac: Slim down the build of 'prefered' ffmpeg snapshot to disable compilation of things we don't need.

Original commit message from CVS:
* configure.ac:
Slim down the build of 'prefered' ffmpeg snapshot to disable compilation
of things we don't need.
* ffmpegrev:
Update our 'prefered' ffmpeg snapshot to rev 15004. This has the fix for a nasty
wma2 decoding regression.