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.
@josevalim make does work on Windows (that's how I build Elixir) but Makefiles written for make typically expect gcc to be installed, which on Windows isn't guaranteed and could create compatibility issues if used.  The motivation behind avoiding make is that if people use a precompiled Elixir package don't need to use MinGW.