Bug: https://bugs.gentoo.org/758167
Full PR is at https://github.com/gentoo/gentoo/pull/32730

Several LLVM packages require this early return, otherwise they fail to 
build on Darwin. I'll also need this USE-flag for 
sys-devel/clang-common, to distinguish between stage2 and stage3 of 
bootstrap-prefix.sh to configure clang differently.

-- 
Best regards,
Alexey "DarthGandalf" Sokolov