Non-optimal build when mli files are missing

[gerdstolpmann]

The build rules are not optimal when mli files are missing, and both bytecode and native code builds are enabled, and the user only requests either a bytecode or native code build. In this case, a special build rule is activated that calls ocamlc and ocamlopt together in order to avoid that partially written cmi files have a bad effect on the build. This is even worse when packs are used in the build (see OCamlPackage and this issue: #73).

There is this ticket for OCaml: https://caml.inria.fr/mantis/view.php?id=7472

Once it is solved there we should revisit the omake build rules, and make the best out of it.