Improve Android support in build system

The idea of this patch to just re-use the C compiler that ocamlc already knows of in `discover.ml`, especially given that the actual compilation is also performed using ocamlc. This allows to use an Android cross-compiler without any additional hacks on lwt side.
> OASIS for whatever reason decides to duplicate the work ocamlfind performs

Until fairly recently ([PR#6300](http://caml.inria.fr/mantis/view.php?id=6300)), ocamlbuild did the wrong thing if you used both `-use-ocamlfind` and `-ocamlc`.