Gappa plugin opam files miss dependency on camlp5
The gappa glugin opam files, e.g. at
(https://github.com/coq/opam-coq-archive/blob/master/released/packages/coq-gappa/coq-gappa.1.4.2/opam)
are missing the dependency on camlp5 so that builds fail unless one happens to have camlp5 installed.