Remove hard dependency on camlp4

This is a preliminary pull request. Discuss, but do not merge yet.
This requirement is due to XOCamlbuildPluginTags. It's possible to inline the required rules from cppo_ocamlbuild directly--they're not large or complex--and retain compatibility with 3.12.

However, if we remove camlp4 requirement, it will even compile green on 3.12 Travis bots!