The libinput backend is now optional. However, this means that a
user building wlroots without the correct libinput dependencies
will end up with a compositor which doesn't respond to input events.
wlr_backend_autocreate is supposed to return a sensible setup, so in
this case let's just error out and explain what happened. Users can
suppress the check by setting WLR_LIBINPUT_NO_DEVICES=1 (already used
to suppress the zero input device case inside the libinput backend).
Compositors which really want to create a bare DRM backend can easily
create it manually instead of using wlr_backend_autocreate.
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