alsa - Add support for media-libs/alsa-lib (Advanced Linux Sound Architecture)
client - Build Normal and QuakeWorld clients alongside the servers
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
flac - Add support for FLAC: Free Lossless Audio Codec
jack - Add support for the JACK Audio Connection Kit
ncurses - Add ncurses support (console display library)