OCaml 4.01.0dev has a Unix.O_CLOEXEC flag, so add this to Lwt_unix too.

This was added quite recently in ocaml/ocaml@2207c45056e69d1ea5
Merge pull request #2 from avsm/fix-4.01-build-with-cloexec

OCaml 4.01.0dev has a Unix.O_CLOEXEC flag, so add this to Lwt_unix too.