configure: Clang build fixes.
I recently noticed problems with building with clang in msvc mode when using my distro clang. It seems specific to its configuration, I think that it doesn't affect default clang.
One problem is that --rtlib=libgcc is specified in /etc/clang, which causes clang to produce:
clang-15: error: unsupported runtime library 'libgcc' for platform 'MSVC'
I think it's a clang bug here: https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChains/MSVC.cpp#L268 It should probably check for -nodefaultlibs in addition to -nostdlib. It's easy to handle on Wine side by using -nostdlib.
The other problem is that clang for some reason doesn't define _MSV_VER by default. I found that it's defined only if I pass -fuse-ld=lld to the command line. That's weird because it shouldn't affect compilation step, it should matter only for linking and that's when we pass it. I didn't track it down in clang code itself, but if we can't depend on clang providing it by default, we need to make sure to pass -fms-compatibility-version ourselves. The exact version doesn't really matter, we mostly need to make sure that it's defined at all so that our #ifdefs work correctly.