this adds an option --with-libcxx, that adds the -stdlib=libc++ flag to link against libc++ instead of libstdc++ on Linux. Currently this is only possible with clang and may not work with all build configurations (e.g. protobuf linked against libstdc++), so this needs to be opt-in rather than being on by default for clang whenever libc++ is detected. Change-Id: Ib4022a58bb2dbd32417c58f01c7443a02ff710fe