diff --git a/config.h.in b/config.h.in
index de225074fb8..d1e0c8f2b45 100644
--- a/config.h.in
+++ b/config.h.in
@@ -70,6 +70,10 @@
    (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