[libcxx] Allow explicit pthread opt-in

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.

http://reviews.llvm.org/D20573

llvm-svn: 270735