av_log: restructure version printing code

Makes the following commit simpler.
av_log: add tons of warnings against mismatched ffmpeg/libav libraries

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.