Add missing definition of SIZE_MAX

If the stdint.h system file follows the ISO C99 specification, it might
not define SIZE_MAX in C++ by default, so provide a local fallback.

gcc/
	* system.h (SIZE_MAX): Define if not already defined.

gcc/system.h

@@ -535,6 +535,10 @@ extern void *realloc (void *, size_t);
 #include <inttypes.h>
 #endif
 
+#ifndef SIZE_MAX
+# define SIZE_MAX INTTYPE_MAXIMUM (size_t)
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif