Autogen: support IMPLIB-only imported target
On Windows, imported shared libraries which only have an
IMPORTED_IMPLIB
set fail if they are depended upon by a target using
automoc. Add a test for the behavior of depending upon all imported
target types from an automoc-using target.
Tested that the test fails without the following commit (just the implib-only
variant changes).
Backport: release
Edited by Brad King