Print a warning if a library has mismatched compile time and link time
versions.
Refuse to work if the compile time and link time versions are a mix of
ffmpeg and libav. We print an error message and call exit(). Since we'd
randomly crash anyway, I think this is ok.
This doesn't catch the case if you e.g. use a ffmpeg libavcodec and a
libav libavformat, which would of course just crash as quickly, but I
think this checks enough already.
This is pretty nasty, because FFmpeg/Libav is yet another library with a
global message callback. We do something with mutexes trying to get it
done, but of course we can't actually solve this problem. If more than
one library in a process use FFmpeg/Libav, only one of them will get log
messages.
Always pass around mp_log contexts in the option parser code. This of
course affects all users of this API as well.
In stream.c, pass a mp_null_log, because we can't do it properly yet.
This will be fixed later.
