ocaml: Enable parallel builds.

Commit f75142c577255b30f2a8e1d27baa5fd185594197 disabled parallel
builds in the ocaml subdirectory (which was the correct thing to do).
This made building the OCaml tests in particular very slow.  Therefore
fix things so that parallel builds can be used again.

See also discussion here about different approaches:

https://www.redhat.com/archives/libguestfs/2014-March/thread.html#00223

This reintroduces commit dce94f3e266ed3f1fc634a1ef6953f2db1510963.