ocamltest Makefile: use ocaml{c,opt}.opt when available
ocamltest Makefile: use ocaml{c,opt}.opt when available

On my machine the sequential build time goes from 7.5s to 3.5s.

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