AVIDump: cleanup
AVIDump: update ffmpeg/libav API usage

libav 10 was released on May 10th, 2014 and it drops support for some
long-deprecated stuff like avcodec_encode_video().
CMake: remove unused/duplicate definitions
CMake: mention ffmpeg in libav messages

We support both.
CMake: update min. libav/ffmpeg library versions
AVIDump: rename frame variables
Merge pull request #362 from Tilka/ffmpeg_libav_new

AVIDump: use new ffmpeg/libav API