berkdb - Add support for sys-libs/db (Berkeley DB)
cli - Add support for CLI interfaces based on readline and libedit
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
gmp - Add support for dev-libs/gmp (GNU MP library)
gui - Enable support for a graphical user interface
ipc - Add support for TIPC inter-process communication