Ben Boeckel authored Jul 01, 2014 and Brad King committed Jul 01, 2014

A library or executable target that consists only of a reference to an
object library may have no sources to compile or custom commands to run.
The command in the target is the link (or archive) command.  Add missing
order-only dependencies to link commands so that target ordering
dependencies are satisfied for it without depending on an intermediate
compilation rule.

Extend the ObjectLibrary test to cover this case.

Co-Author: Brad King <brad.king@kitware.com>