This allows us to build with libdecor enabled as the selection to use it
is decided upon at runtime if the compositor `gnome-shell` is detected.
If the libdecor development headers are installed, by default it will
now be compiled in unless overridden by the user at compile time.
`$DISPLAY` will be set even in a Wayland session, which causes LG to
initialize itself under Xwayland unless it is explicitly compiled with
`-DENABLE_X11=OFF`.
We could add a Wayland check within the X11 backend, but reordering the
code-generated array seems like a better solution.
