Makefile.nt: make sure flexdll is installed only under Windows
Makefile.nt: make sure flexdll is installed only under Windows

This has no effect now but will have once the install rule will be shared.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml