unix: only build the Lwt_unix subpackage if lwt.unix is detected

On a mirage-xen system, there won't be an lwt.unix

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add dependency on cmdliner

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #2 from djs55/detect-unix

unix: only build the Lwt_unix subpackage if lwt.unix is detected