Respect wayland configure flags

Don't attempt to use the WAYLAND_SCANNER macro even if it is enabled
unless the user explicitly asks to enable wayland support.

--- configure.ac.old	2018-04-17 16:39:23.835077504 -0700
+++ configure.ac	2018-04-17 16:41:01.474678546 -0700
@@ -168,13 +168,13 @@
     PKG_CHECK_MODULES([LIBVA_WAYLAND_DEPS], [libva-wayland],
         [AC_DEFINE([HAVE_VA_WAYLAND], [1], [Defined to 1 if VA/Wayland API is enabled])],
         [USE_WAYLAND="no"])
+
+    m4_ifdef([WAYLAND_SCANNER_RULES],
+        [WAYLAND_SCANNER_RULES(['$(top_builddir)/src/wayland'])],
+        [wayland_scanner_rules=""; AC_SUBST(wayland_scanner_rules)])
 fi
 AM_CONDITIONAL(USE_WAYLAND, test "$USE_WAYLAND" = "yes")
 
-m4_ifdef([WAYLAND_SCANNER_RULES],
-    [WAYLAND_SCANNER_RULES(['$(top_builddir)/src/wayland'])],
-    [wayland_scanner_rules=""; AC_SUBST(wayland_scanner_rules)])
-
 AC_OUTPUT([
     Makefile
     debian.upstream/Makefile