[Coq8.4pl1] Add dependency on camlp5.

Camlp5 seems preferred to camlp4. Moreover other packages make the implicit assumption that camlp5 is used for compiling coq (eg. why3).

I used "%{lib}%/camlp5" for specifying the library directory of camlp5 (coq doesn't use ocamlfind). Is there a better way? No opam variable  seems to give the installation directory of a package (eg. camlp5:lib ).

Best,