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>

configure.ac

@@ -1730,6 +1730,13 @@ if test "x$with_egl_platforms" != "x" -a "x$enable_egl" != xyes; then
     AC_MSG_ERROR([cannot build egl state tracker without EGL library])
 fi
 
+PKG_CHECK_MODULES([WAYLAND_SCANNER], [wayland_scanner],
+        WAYLAND_SCANNER=`$PKG_CONFIG --variable=wayland_scanner wayland_scanner`,
+        WAYLAND_SCANNER='')
+if test "x$WAYLAND_SCANNER" = x; then
+    AC_PATH_PROG([WAYLAND_SCANNER], [wayland-scanner])
+fi
+
 # Do per-EGL platform setups and checks
 egl_platforms=`IFS=', '; echo $with_egl_platforms`
 for plat in $egl_platforms; do
@@ -1737,9 +1744,9 @@ for plat in $egl_platforms; do
 	wayland)
 		PKG_CHECK_MODULES([WAYLAND], [wayland-client >= $WAYLAND_REQUIRED wayland-server >= $WAYLAND_REQUIRED])
 
-                WAYLAND_PREFIX=`$PKG_CONFIG --variable=prefix wayland-client`
-                AC_PATH_PROG([WAYLAND_SCANNER], [wayland-scanner],,
-                             [${WAYLAND_PREFIX}/bin$PATH_SEPARATOR$PATH])
+		if test "x$WAYLAND_SCANNER" = x; then
+			AC_MSG_ERROR([wayland-scanner is needed to compile the wayland egl platform])
+		fi
 		;;
 
 	x11)