https://slackbuilds.org/repository/15.0/multimedia/x264/ を参照して作業。
# wget https://slackbuilds.org/slackbuilds/15.0/multimedia/x264.tar.gz
# tar xf x264.tar.gz
# cd x264
# wget https://sources.liwjatan.org/x264/x264-git_20210613_5db6aa6c.tar.lz
# ./x264.SlackBuild
# installpkg /tmp/x264-git_20210613_5db6aa6c-x86_64-1_SBo.tgz