Msys2 Makefile.meson: Initial clang64 support
We cannot have full msys2 installer builds with clang64 yet (there's no nsis), but freeciv build part with Meson works, so add initial clang64 support to Makefile.meson
We cannot have full msys2 installer builds with clang64 yet (there's no nsis), but freeciv build part with Meson works, so add initial clang64 support to Makefile.meson