Fixes the path to campl4

From version 4.02 of OCaml, camlp4 is not distributed as part of the
OCaml release. Therefore the camlp4 libraries may be installed in a
directory different from `+camlp4`.

This patch uses ocamlfind to discover where camlp4 is actually
installed.
Merge pull request #3 from vbgl/master

Fixes the path to campl4