This patch attempts to fix the libc++ build/link so that it doesn't use an default C++ libraries on Windows. This is needed to prevent linking to MSVC's STL library.
Additionally this patch changes libc++ so that it is always linked with the non-debug DLL's (e.g. /MD). This is needed so that the test suite can correctly link the same libraries without needing to know which configuration c++.dll was linked with.
Not the biggest fan of this name, since it's not obvious why MinGW shouldn't count as targeting Windows. I thought of LIBCXX_TARGETING_NATIVE_WINDOWS or LIBCXX_TARGETING_MSVCRT instead, but MinGW is also native Windows and targets MSVCRT, so those names aren't any better from that perspective either. I can't think of anything else at the moment, but I'm sure there's a better name.