OCamlbuild: Fix the check of ocamlfind

As noticed in the following issues, ocamlbuild checks if ocamlfind is present too early (as it should be done after the After_options hook).
https://github.com/ocaml/opam-repository/pull/1641
https://github.com/ocaml/camlp4/issues/14

However, it appeared to be a difficult problem because of plugin tags. So a warning still appears if ocamlfind has been disabled in the plugin since ocamlfind is used to build the plugin itself.

I've also added a little patch that fixes a shadowing bug (maybe I should have done another PR for that).