CMake Nightly Date Stamp
Allow target_link_libraries with IMPORTED targets.

This makes it possible to use:

 target_link_libraries(foo LINK_INTERFACE_LIBRARIES bar)

where foo is an IMPORTED target. Other tll() signatures are not
allowed.
Merge topic 'tll-IMPORTED-targets' into next

9cfe4f1 Allow target_link_libraries with IMPORTED targets.
97758f2 CMake Nightly Date Stamp