2.0 beta More BSD work

FreeBSD 9 at least, not sure about 10, needs WITH_LIBCPLUSPLUS to build and install the LLVM libc++, document this.

Furthermore, all three major BSDs require linking with libpthread to pick up any of the pthread_*() functions. I think I got the right gmake-fu applied to the Makefile and also in a pretty clean way that doesn't clutter the rest of the file up too much.