[MPlayer-dev-eng] FFmpeg build system integration

[Diego Biurrun]

As you may have noticed the MPlayer build is currently broken due to the
latest changes to the build system in FFmpeg.  Mans came up with the
simple but clever idea of putting all the common stuff from the
different libav* Makefiles into a file in the root of the src tree:
common.mak.

While this shaves more than 150 lines from the FFmpeg Makefiles, it's a
problem for us since we need to integrate this file now in one form or
the other.  We have several options:

* Copy common.mak to our source tree similar to the libav* dirs.
* cvs add common.mak to our source tree.
* Put all of ffmpeg in our source tree instead of just a few dirs.

The last is probably the cleanest, but it would add about 1.5M of code.
I wouldn't say this is a big problem, though.  We can remove it from
releases and snapshots and CVS users have to download all FFmpeg
anyway..

Diego