Make camlp4 insensitive to CDPATH.

Ref: https://github.com/ocaml/opam-repository/issues/2561.

This is the second time I encounter this bug, and I've seen people on IRC occasionally come in with the same report. I believe there are no downside to these changes, and while the problem is fairly obscure, I think this is worth merging.

It's interesting that practically nothing except camlp4 uses cd in build scripts.