Sorry for reviving an old topic, but I think it makes sense.
I have the same issue, only in my case the included file is not a sibling but a few directories above - meaning I can’t use CMAKE_CURRENT_LIST_DIR as starting point. I’d really want to specify it from CMAKE_SOURCE_DIR instead.
Is this possible? If not, why not?
Edit: I can do something like ${CMAKE_CURRENT_LIST_DIR}../../../myfile.cmake so I guess the question “why” I can’t base this off off CMAKE_SOURCE_DIR still stands?