Use experimental cppo ocamlbuild plugin.

Use the ocamlbuild plugin for cppo proposed here: https://github.com/mjambon/cppo/pull/4
It's only slightly more simple, but it made me fix a bug in the include tag, so it's good.

You need to invoke ocamlbuild with the argument `-plugin-tag "package(cppo_ocamlbuild)"` (in top of `-use-ocamlfind`). This argument is only available in ocaml >= 4.01, so you may not want to actually merge this. :)