ocamlbuild: when tag "native" is set, use ocamlopt instead of ocamlc for .mli->.cmi

This was requested (with a patch proposal) by "jessicah" in
PR#4613. Given that the effect of such changes are hard to test,
I commit this in trunk only, not version-4.01, to give more time to
detect eventual problems.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13787 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02