Move down the INT_MAX logic, because HAVE_LIMITS_H was always undefined

and this breaks the AIX build with an INT_MAX redefinition error.
"config.h" is included in pgenheaders.h, so moving this down fixes the
problem.

Parser/node.c

 
+/* Parse tree node implementation */
+
+#include "pgenheaders.h"
+#include "node.h"
+#include "errcode.h"
+
 #ifdef HAVE_LIMITS_H
 #include <limits.h>
 #endif
@@ -6,12 +12,6 @@
 #define INT_MAX 2147483647
 #endif
 
-/* Parse tree node implementation */
-
-#include "pgenheaders.h"
-#include "node.h"
-#include "errcode.h"
-
 node *
 PyNode_New(int type)
 {