Error if linked target has relative paths in INTERFACE_INCLUDE_DIRECTORIES

We can do this check only if the TargetName is non-empty, which means
that we're evaluating INTERFACE_INCLUDE_DIRECTORIES from a linked
dependency which was set using target_link_libraries.

It is possible to have relative paths in INCLUDE_DIRECTORIES already
in CMake 2.8.10.2, so that part will require a policy to fix.