autotools: Fix AC_PATH_PROG call

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
diff --git a/configure.ac b/configure.ac
index 0129157..43e5e5c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -424,8 +424,10 @@
 fi
 AM_CONDITIONAL(HAVE_LCMS, [test "x$have_lcms" = xyes])
 
-AC_PATH_PROG([wayland_scanner], [wayland-scanner],
-	     [AC_MSG_ERROR("wayland-scanner is needed to compile weston")])
+AC_PATH_PROG([wayland_scanner], [wayland-scanner])
+if test x$wayland_scanner = x; then
+	AC_MSG_ERROR([wayland-scanner is needed to compile weston])
+fi
 
 AC_CONFIG_FILES([Makefile
 		 shared/Makefile