ocaml related ports cannot be built in parallel

[garrigue@…]

Most ocaml related ports cannot be built in parallel, as they implicitly expect make to call dependencies sequentially.

I had to add the option "use_parallel_build no" manually to camlp5 and coq, but this would probably a good idea to all ports depending on lang/ocaml.

[jmr@…]

camlp5 has had parallel build disabled since r57526, and coq explicitly sets use_parallel_build yes, so presumably it works that way.