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
eigen - Enable dev-cpp/eigen mathematical templates support
nls - Add Native Language Support (using gettext - GNU locale utilities)
test - Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
