configure: rework wayland_scanner handling(fix make distcheck)

configure: rework wayland_scanner handling(fix make distcheck)

Currently having the wayland-scanner is optional, which causes problems
when autotools parses through the makefiles, and tries to generate all
the BUILT_SOURCES.

As the config option --with-egl-platform=wayland is not the default, we
won't end up setting the WAYLAND_SCANNER variable, which in turn will
cause some files to not get generated.

There has been a wayland-scanner package as of wayland 1.2 which
provides a variable for the scanner binary, so let's use that one and
fall back to manually searching via AC_PATH_PROG when needed.

Cc: "10.5" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>