The existing pthread detection code in __config is pretty good for common operating systems. It doesn't allow cmake-time choices to be made for uncommon operating systems though.
This change adds the LIBCXX_HAS_PTHREAD_API cmake flag, which turns into the _LIBCPP_HAS_THREAD_API_PTHREAD preprocessor define. This is a name change from the old _LIBCPP_THREAD_API_PTHREAD. The lit tests want __config_site.in variables to have a _LIBCPP_HAS prefix.