Search both INTERFACE_LINK_LIBRARIES and LINK_LIBRARIES.

This allows us to enable assert_target_is_linked_to on travis again,
where libraries specified in LINK_LIBRARIES were not automatically
propogated to INTERFACE_LINK_LIBRARIES
Merge pull request #30 from smspillaz/cmake-unit.item_28

Search both INTERFACE_LINK_LIBRARIES and LINK_LIBRARIES.