avcommon: use a specific macro to check the FFmpeg libavcodec version

avcommon: use a specific macro to check the FFmpeg libavcodec version

This macro doesn't check for libav which is assumed to not have to relevant
code. This is the same macro name used in VLC 4.0.

Gbp-Pq: Name 0087-avcommon-use-a-specific-macro-to-check-the-FFmpeg-li.patch