Avoid install prefix in config path lookup
CMAKE_INSTALL_FULL_SYSCONFDIR has unexpected behaviour when used with prefixes.
read-only single-machine data (etc, or /etc if CMAKE_INSTALL_PREFIX is /usr)
This is typically because prompting for root every install would be annoying. Our use case here is not to install files but to refer to a file in a known location once installed, so we do not want this behaviour.