Fix sysrooted MinGW header search paths

Attached is an updated patch which adds support for the
i686-w64-mingw32 paths as well. This patch modifies
AddMinGW64CXXIncludePaths to work more like the non-*64* variant. This
expects CLang to be installed in the exact same place as a sysrooted
mingw-w64 GCC.

Comments or commits are welcome!

Ruben

patch.txt (5.35 KB)