berkdb - Add support for sys-libs/db (Berkeley DB for MySQL)
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
gdbm - Add support for sys-libs/gdbm (GNU database libraries)
ithreads - Enable Perl threads, has some compatibility problems
minimal - Install a very minimal build (disables, for example, plugins, fonts, most drivers, non-critical features)
quadmath - Enable 128bit floating point arithmetic via libquadmath