curl_ntlm_core.c: use the limits.h's SIZE_T_MAX if provided

lib/curl_ntlm_core.c

@@ -646,11 +646,14 @@ CURLcode Curl_hmac_md5(const unsigned char *key, unsigned int keylen,
   return CURLE_OK;
 }
 
+#ifndef SIZE_T_MAX
+/* some limits.h headers have this defined, some don't */
 #if defined(SIZEOF_SIZE_T) && (SIZEOF_SIZE_T > 4)
 #define SIZE_T_MAX 18446744073709551615U
 #else
 #define SIZE_T_MAX 4294967295U
 #endif
+#endif
 
 /* This creates the NTLMv2 hash by using NTLM hash as the key and Unicode
  * (uppercase UserName + Domain) as the data