If you scroll down to [Special Cases](https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html), you can see that it describes what you want. I may have an error on how it's evaluating the path, I'll look into that.
I see that the command you used produced this: `-DCMAKE_INSTALL_PREFIX=/usr` which is a special case and should do what you describe by changing `${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_SYSCONFDIR}` to `${CMAKE_INSTALL_FULL_SYSCONFDIR}/`.
Quick, question though. Is there a case where you will want to install in a specific folder and then install the cfg somewhere else? (Apart from the special case described in the link.)
I don't think we should configure or define CFG_DIR; it should be evaluated from the install prefix according to the rules described above.