[PATCH] [libc++] Allow libc++ to be built on systems without POSIX threads

[Marshall Clow]

In general, libc++ uses the negative for missing capabilities.
_LIBCPP_HAS_NO_RVALUE_REFERENCES
_LIBCPP_HAS_NO_ADVANCED_SFINAE
_LIBCPP_HAS_NO_VARIADICS
_LIBCPP_HAS_NO_TRAILING_RETURN

I would prefer to keep this convention.

http://reviews.llvm.org/D3969