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.
Quick question: does make ever work on Windows? I am asking because maybe we should check if `nmake` is available first? Or at least check if `Makefile.win` is available? Isn't there a chance a makefile will work fine on both OS?