If a non-IMPORTED library is added to the INTERFACE_LINK_LIBRARIES
of a IMPORTED target, the non-IMPORTED target needs to become a
target dependency and link dependency of the consuming target.
This is already the case since commit 30962029 (Make targets depend
on the link interface of their dependees, 2012-12-26), and fixed in
the parent commit, so test that it works.
Stephen Kelly