Polkit1Backend: build also w/o Wayland support in KWindowSystem
KWindowSystem can be built without Wayland support, and in that case
KWaylandExtras is not available. Use __has_include()
to detect whether
KWaylandExtras is available, and enable the Wayland bits only in that
case. KWindowSystem is a required dependency of the Polkit1 backend,
so it is possible to assume that the lack of KWaylandExtras means that
KWindowSystem was built without Wayland support.