Start using flexible array members

Flexible array members are a C99 feature that avoids "cheating" in the
declaration of variable-length arrays at the end of structs.  With
Autoconf support, this should be transparent for older compilers.

We start with one use in gist.h because gcc 4.6 started to raise a
warning there.  Over time, it can be expanded to other places in the
source, but they will likely need some review of sizeof and offsetof
usage.  The current change in gist.h appears to be safe in this
regard.

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

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

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))

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