build errors with MinGW-W64 GCC-8.1.0 on Windows

Hi all,

You know, as the compile errors all were in thread_win32.cpp I was
suspicious how the build would do with x86_64-win32-seh but that failed
because Mingw-w64 did not have the header file mutex.

As I have the llvm libcxx library with this mutex header file (among other
stuff) would there be a way to have Mingw-w64 use this library during the
build of llvm and clang or wouldn't that work?

Also is there any documentation for specifics in building llvm with
Mingw-w64?

Regards, Maarten