So when installing an rpm there is a file collision check. When you add a file to an rpm package with cmake / cpack it unfortunately adds parent directories your file goes into, which will give you conflicts with other packages. But well you have that beautiful feature to exclude directories from being added:
CPACK_RPM_EXCLUDE_FROM_AUTO_FILELIST_ADDITION
Now somehow it failed. Turned out it would not work if my path ended with a backslash. Brush my banana! Like "/etc/sysctl.d" is a different animal than "/etc/systclt.d/". - But at least that's nothing against the strangeness of the "mv" command in those respects.