According to the first comment for this question, and i quote
"Libraries with this name are provided from both the FFmpeg project and the Libav project, but they are mutually incompatible". So I think the answer depends on which libavcodec do you want to use.
What are the differences that exist between this 2 projects ?
I'm interested in video and audio encoding to H/X264, Mp3 and AAC with Mp4/Mkv containers; but more importantly i would like to start with a reliable and up to date library with a permissive license and widely supported on as many platforms as possible.
The most visible differences between ffmpeg and the fork for general users are the different binary names (ffmpeg, ffplay, ffprobe vs avconv, avprobe, avplay etc). FFmpeg merges most commits from libav, but it's not as reciprocal, so libav does not have as many features. Therefore, ffmpeg can do most of what libav can, but libav can't always do everything ffmpeg can.