Incorporate libc++

[phillipjohnston]

The mutex examples we give require some files that are only found in clang's libc++. We could add a dependency for these specific examples which provides the missing files when you're using GCC (e.g., similar to embvm)

Maybe this is really best addressed by #46, since we can just direct to our implementation in the C++ library. We can have our dispatch example use that mutex... or just the built-in one.