cally: Fix compilation without X11 backend

Commit 0fd9e381 fixed setting the out parameter for the x coordinate
when using the X11 backend, but broke compilation when the backend is
not available ...
Really fix the issue by running the X11-specific code when the X11
backend is available and in use, and display the one-time warning
otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=781902