My method was to find the LIBAVUTIL_VERSION_* at the commit where the
updated AV_ prefixed constants became available, add one "micro" version
to it, then if it's not at least that version, provide the unprefixed
constant. It just so happens that the same commit introduced all
the AV_* constants.
The AV_ prefixed constants have been available since
ffmpeg@def97856de6021965db86c25a732d78689bd6bb0 (2015-07-07), or version
2.8.
The non-prefixed constants are set to be removed in the next ffmpeg
release, and were removed in
ffmpeg@302554835e39b79b977ed60c9afe81b44590dfef (2016-12-05).
