OK, after further investigation, there are no files
remaining installed, but only empty folders.
Sorry, I did only do an `ls` on the root project folder,
instead of a `find` to list all files :(
Therefore I do not seem to have an error in my
spec file. I only need a possibility to clean out
empty folders when updating the package.
Even when a clean function/macro doesn't exist, this
might not lead to errors during project life-time, as I
can run a line of code in %post or writing a hint into
the project documentation.