atomic-builtins - Compile atomic builtins into compiler-rt.builtins. This is needed on systems without GCC/libatomic, but requires using Clang to build.
clang - Force building using installed clang (rather than the default CC/CXX).
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
test - Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)
