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
lirc - Add support for lirc (Linux's Infra-Red Remote Control)
video_cards_nvidia - VIDEO_CARDS setting to build driver for nvidia video cards
