configure.ac: provide a fall-back define for WAYLAND_SCANNER
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 <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler