def.h: Provide hton* / ntoh* functions to users by default for compatibility (can be turned off)

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UPGRADING

@@ -49,6 +49,10 @@ with newer versions.
     * Added generalized abstraction for itoa(), strnicmp(), stricmp() and strnstr()
       in def.h (to be overridden in cc.h) instead of config 
       options for netbiosns, httpd, dns, etc. ...
+    * New abstraction for hton* and ntoh* functions in def.h.
+      To override them, use the following in cc.h: 
+      #define lwip_htons(x) <your_htons>
+      #define lwip_htonl(x) <your_htonl>
 
   +++ new options:
      * TODO

src/include/lwip/def.h

@@ -92,6 +92,14 @@ u32_t lwip_htonl(u32_t x);
 #endif
 #define lwip_ntohl(x) lwip_htonl(x)
 
+/* Provide usual function names as macros for users, but this can be turned off */
+#ifndef LWIP_DONT_PROVIDE_BYTEORDER_FUNCTIONS
+#define htons(x) lwip_htons(x)
+#define ntohs(x) lwip_ntohs(x)
+#define htonl(x) lwip_htonl(x)
+#define ntohl(x) lwip_ntohl(x)
+#endif
+
 /* These macros should be calculated by the preprocessor and are used
    with compile-time constants only (so that there is no little-endian
    overhead at runtime). */