make manager calls nmake /F Makefile.win on Windows

Dependencies on Windows should compile with `nmake /F Makefile.win`.  So these commits set `:make` as the manager if either `Makefile` OR `Makefile.win` exist, then makes the platform-appropriate call.
Ok so I think we should definitely check if there is a `Makefile.win` before invoking `nmake \F Makefile.win` for now. I will amend it locally.