x11 backend of the touchpad KCM uses xserver-properties.h that is part of
xorg-server package. As XORG_INCLUDE_DIRS seems to be empty, so far, it was
relying on Synaptics_INCLUDE_DIRS implicitly and worked only if the header
happened to be in the same directory as Synaptics' which is the case most of
FindXorgServer.cmake is just a copy of FindXorgLibinput, adapted accordingly.