Remove `ounit` as as an unnecessary opam dependency
Make camlp4 an optional dependency.

Note that this will require checking all the downstream consumers
of OPAM cstruct and adding camlp4 explicitly.
Merge branch 'camlp4-optional' of git://github.com/avsm/ocaml-cstruct into avsm-camlp4-optional

Conflicts:
	_oasis
	opam
	setup.ml
bump to 1.5.0