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.
`-ocamlc $ocamlc` is removed, as that option is nearly never used, and OASIS for whatever reason decides to duplicate the work ocamlfind performs. Specifically, this breaks `OCAMLFIND_TOOLCHAIN` support (see https://github.com/whitequark/ocaml-findlib/commit/d6e6afdbd5b1d0271a01c0c8aac7a83555ee7213).