Use bundled libc++ instead of libstdc++ for building.

Packages describing “libcxx” as local USE flag

Package “libcxx” Flag Description
www-client/chromium Use bundled libc++ instead of libstdc++ for building.
sys-devel/clang-runtime Install sys-libs/libcxx for -stdlib=libc++
sci-physics/root Build ROOT against sys-libs/libcxx

All packages providing a “libcxx” USE flag (3)