--- a/config.m4
+++ b/config.m4
@@ -38,7 +38,7 @@
 PHP_ARG_ENABLE([gd],
   [for GD support],
   [AS_HELP_STRING([--enable-gd],
-    [Include GD support])])
+    [Include GD support])], no, no)
 
 if test "$PHP_GD" != "no"; then