ocamlbuild: make it possible to change the ocamlmklib command

Add a -ocamlmklib option.
Use ocamlfind for ocamlmklib as well, when told to do so.

This is useful when crosscompiling, to select an alternative ocamlmklib
that produces libraries for the target platform.

From: Jerome Vouillon <jerome.vouillon@pps.univ-paris-diderot.fr>

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15532 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02