Add support for libcxx

Packages describing “libcxx” as local USE flag

Package “libcxx” Flag Description
sys-devel/binutils-apple Add support for libcxx
sci-physics/root Build ROOT against sys-libs/libcxx
sys-devel/clang-runtime Install sys-libs/libcxx for -stdlib=libc++

All packages providing a “libcxx” USE flag (2)