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
e7d7199ea4