configure.ac: make use of wayland-scanner.pc

Replace the current (incorrect) assumption that wayland-scanner is
located in the wayland-client prefix. Make use of the wayland_scanner
variable in wayland-scanner.pc

It was introduced back in 2013 and we already require newer wayland bits

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>