Marble built with -DCMAKE_INSTALL_PREFIX=/usr installs its data into /usr/etc/xdg/marble.knsrc, what is wrong.
This is because KDEInstallDirs do not handle special case of CMAKE_INSTALL_SYSCONFDIR=etc with CMAKE_INSTALL_PREFIX=/usr.
I am not sure where CMAKE_INSTALL_SYSCONFDIR is set, but certainly not from commandline. Probably from GNUInstallDirs.
GNUInstallDirs already handle special cases: let's use it.
This change complements https://cgit.kde.org/extra-cmake-modules.git/commit/?id=e92da33 and make behavior as documented, i.e. :
read-only single-machine data (etc, or /etc if CMAKE_INSTALL_PREFIX is /usr)