Cherry-pick r15412 from trunk to fix #6592 (Prepare environment (with Compenv.readenv) before executing script passed on the toplevel command-line.).

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15416 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02