another reasonably simple option would be to have a manual step, and copy what happens with apple-gcc42 +bootstrap.
We make up a non-TLS variant of libcxx, call it libcxx +bootstrap, and default to installing that if there is nothing in /usr/lib/libc++.dylib. Then the port notes (like apple-gcc42) would tell people what to do: require the installation of clang-5.0, then run: sudo port install libcxx -bootstrap and that gets the binary installed. From then on, people would just have to sudo port install libcxx and they would get the TLS version. Ken
