build/generate: Ignore printf format warnings for WINHOST=y

%I64d is correct, even if the compiler complains about it sometimes
build/util: Added slash-subst macro

Calling ./foo/bar.exe is not possible on Windows...
it has to be .\foo\bar.exe
build/util: Added create-directory macro

This standard Windows mkdir.exe does not know about the -p option. The
GnuWin32 mkdir.exe understands it but needs to be called explicitly.