Before this patch, the following is reported falsely as a self-reference:

 target_link_libraries(empty2 LINK_PUBLIC empty3)
 target_link_libraries(empty3 LINK_PUBLIC empty2)

 add_custom_target(...
    -DINCLUDES=$<TARGET_PROPERTY:empty2,INTERFACE_INCLUDE_DIRECTORIES>
 )

The reason is that the existing code assumed that all reading of
include directories would be done through cmTarget::GetIncludeDirectories()
and would therefore be initialized with a DagChecker. That is not the case
if reading the property with an 'external' generator expression.