diff --git a/configure b/configure
index a2c3aab407..9b8cb48931 100755
--- a/configure
+++ b/configure
@@ -14954,6 +14954,77 @@ _ACEOF
 
 fi
 
+
+  { $as_echo "$as_me:$LINENO: checking for flexible array members" >&5
+$as_echo_n "checking for flexible array members... " >&6; }
+if test "${ac_cv_c_flexmember+set}" = set; then
+  $as_echo_n "(cached) " >&6
+else
+  cat >conftest.$ac_ext <<_ACEOF
+/* confdefs.h.  */
+_ACEOF
+cat confdefs.h >>conftest.$ac_ext
+cat >>conftest.$ac_ext <<_ACEOF
+/* end confdefs.h.  */
+#include <stdlib.h>
+	    #include <stdio.h>
+	    #include <stddef.h>
+	    struct s { int n; double d[]; };
+int
+main ()
+{
+int m = getchar ();
+	    struct s *p = malloc (offsetof (struct s, d)
+				  + m * sizeof (double));
+	    p->d[0] = 0.0;
+	    return p->d != (double *) NULL;
+  ;
+  return 0;
+}
+_ACEOF
+rm -f conftest.$ac_objext
+if { (ac_try="$ac_compile"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:$LINENO: $ac_try_echo\""
+$as_echo "$ac_try_echo") >&5
+  (eval "$ac_compile") 2>conftest.er1
+  ac_status=$?
+  grep -v '^ *+' conftest.er1 >conftest.err
+  rm -f conftest.er1
+  cat conftest.err >&5
+  $as_echo "$as_me:$LINENO: \$? = $ac_status" >&5
+  (exit $ac_status); } && {
+	 test -z "$ac_c_werror_flag" ||
+	 test ! -s conftest.err
+       } && test -s conftest.$ac_objext; then
+  ac_cv_c_flexmember=yes
+else
+  $as_echo "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+	ac_cv_c_flexmember=no
+fi
+
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:$LINENO: result: $ac_cv_c_flexmember" >&5
+$as_echo "$ac_cv_c_flexmember" >&6; }
+  if test $ac_cv_c_flexmember = yes; then
+
+cat >>confdefs.h <<\_ACEOF
+#define FLEXIBLE_ARRAY_MEMBER /**/
+_ACEOF
+
+  else
+    cat >>confdefs.h <<\_ACEOF
+#define FLEXIBLE_ARRAY_MEMBER 1
+_ACEOF
+
+  fi
+
 { $as_echo "$as_me:$LINENO: checking for signed types" >&5
 $as_echo_n "checking for signed types... " >&6; }
 if test "${pgac_cv_c_signed+set}" = set; then
diff --git a/configure.in b/configure.in
index ddc4cc9a16..e873c7b157 100644
--- a/configure.in
+++ b/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 a/src/include/access/gist.h b/src/include/access/gist.h
index df9f39c7b8..5ce7325ff9 100644
--- a/src/include/access/gist.h
+++ b/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 a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 5d38f25d26..19f38cc0fd 100644
--- a/src/include/pg_config.h.in
+++ b/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