WIP: When built on s390x, replace the gdm library with a stub
gdm and gnome-shell are unavailable on s390x, but if we want to install gnome-session-flashback as an alternative to gnome-shell, we'll need gnome-panel.
This compiles on s390x. I haven't tested it (I couldn't get XDMCP from a s390x qemu VM to work, with either lightdm or xdm).