Draft: Small enhancements for Pango on Windows

TODO: would be great to remove the dependency on libstdc++-6.dll (GCC) or libc++.dll (CLang), but doesn't seem quite possible at the moment: -static-libstdc++ is available on mingw GCC but doesn't have any effect, CLang has no equivalent.

We may consider using the Hybrid CRT on MSVC, but maybe it's not that important...