diff --git i/configure.in w/configure.in
index ddc4cc9..e873c7b 100644
--- i/configure.in
+++ w/configure.in
@@ -1110,6 +1110,7 @@ AC_C_BIGENDIAN
 AC_C_CONST
 PGAC_C_INLINE
 AC_C_STRINGIZE
+AC_C_FLEXIBLE_ARRAY_MEMBER
 PGAC_C_SIGNED
 AC_C_VOLATILE
 PGAC_C_FUNCNAME_SUPPORT
diff --git i/src/include/access/gist.h w/src/include/access/gist.h
index df9f39c..f3dfcaa 100644
--- i/src/include/access/gist.h
+++ w/src/include/access/gist.h
@@ -144,7 +144,7 @@ typedef struct GISTENTRY
 typedef struct
 {
 	int32		n;				/* number of elements */
-	GISTENTRY	vector[1];		/* variable-length array */
+	GISTENTRY	vector[FLEXIBLE_ARRAY_MEMBER];
 } GistEntryVector;
 
 #define GEVHDRSZ	(offsetof(GistEntryVector, vector))
diff --git i/src/include/pg_config.h.in w/src/include/pg_config.h.in
index 5d38f25..19f38cc 100644
--- i/src/include/pg_config.h.in
+++ w/src/include/pg_config.h.in
@@ -61,6 +61,15 @@
    (--enable-thread-safety) */
 #undef ENABLE_THREAD_SAFETY
 
+/* 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
+
 /* float4 values are passed by value if 'true', by reference if 'false' */
 #undef FLOAT4PASSBYVAL