configure.ac: provide a fall-back define for WAYLAND_SCANNER

In some cases, we can end up calling WAYLAND_SCANNER even when
there's no binary. Do follow the other's approach set by
AX_PROG_FLEX/BISON and set the variable to :

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
1 file changed