Use system x265 (patch from Gentoo).
Use system libvpx (patch from FreeBSD).
Use system ffmpeg (patch from upstream).
Add fdk_acc build option (off).
Close#3987.
"The licenses of libfaac and libfdk_aac are not compatible with the GPL,
so the GPL does not permit distribution of binaries containing code
licensed under these licenses when GPL-licensed code is also included.
Therefore these encoders have been designated as "non-free", and you
cannot download a pre-built ffmpeg that supports them. This can be
resolved by compiling ffmpeg yourself."
See https://trac.ffmpeg.org/wiki/Encode/AAC.
That means that version comparators are not supported anymore in
hostmakedepends and makedepends.
This will ensure that a pkg is always built with the same build dependencies
everywhere, if the srcpkgs tree is uptodate.