diff --git a/acconfig.h b/acconfig.h
index feddb31e7f7..69a8634c249 100644
--- a/acconfig.h
+++ b/acconfig.h
@@ -9,6 +9,10 @@
    (as it does on SCI ODT 3.0) */
 #undef BAD_STATIC_FORWARD
 
+/* Define for AIX if your compiler is a genuine IBM xlC/xlC_r
+   and you want support for AIX C++ shared extension modules. */
+#undef AIX_GENUINE_CPLUSPLUS
+
 /* Define this if you have BeOS threads */
 #undef BEOS_THREADS