Locate "topfind" when using opam "system" compiler.

When using opam with the "system" compiler the "topfind" file is not
found by the toplevel.  However, the environment variable
$OCAML_TOPLEVEL_PATH is set. See
https://github.com/OCamlPro/opam-repository/issues/34
Merge pull request #12 from Chris00/master

Locate "topfind" when using opam "system" compiler.