diff --git a/includes/config.h.in b/includes/config.h.in
index fa040918..3b1cf564 100644
--- a/includes/config.h.in
+++ b/includes/config.h.in
@@ -22,6 +22,15 @@
 /* Define to include Failover Protocol support. */
 #undef FAILOVER_PROTOCOL
 
+/* Define to nothing if C supports flexible array members, and to 1 if it does
+   not. That way, with a declaration like `struct s { int n; double
+   d[FLEXIBLE_ARRAY_MEMBER]; };', the struct hack can be used with pre-C99
+   compilers. When computing the size of such an object, don't use 'sizeof
+   (struct s)' as it overestimates the size. Use 'offsetof (struct s, d)'
+   instead. Don't use 'offsetof (struct s, d[0])', as this doesn't work with
+   MSVC and with C++ compilers. */
+#undef FLEXIBLE_ARRAY_MEMBER
+
 /* Define to 1 to use the Berkeley Packet Filter interface code. */
 #undef HAVE_BPF