[PATCH] D20328: [libcxx] Externally threaded libc++ variant

[Stephan T. Lavavej via cfe-commits]

[Asiri Rathnayake]
> The `constexpr` mutex constructor is apparently also a
> pain point on windows [1], I'm sure @STL_MSFT knows more about it.

That's because of MSVC-specific issues, where we need to dynamically switch between WinAPI and ConcRT implementations depending on whether the end user's machine is Win7+/Vista/XP. If we could assume Win7+, we wouldn't need this squirrelly machinery.

STL