Followed Steve's good recommendation, and made routed_get_my_address() be a variant of datalink_get_my_address() just when routing is in use. Haven't done anything about the sending functions yet.

Renamed Lookup_Routed_Device_Address() as Routed_Device_Address_Lookup(), and replaced it in the routed npdu handler with a function that finds the "next" gateway or routed device, as per the destination address.  (Less tied to the specifics of the gw_device.c implementation.)
Fixed a few build warnings (eg, %zu for size_t arguments).
Until we improve the makefile system, I've enabled BAC_ROUTING in config.h

bacnet-stack/demo/handler/s_readrange.c

@@ -72,11 +72,7 @@ uint8_t Send_ReadRange_Request(
 
     if (invoke_id) {
         /* encode the NPDU portion of the packet */
-#if BAC_ROUTING
-        my_address = *Get_Routed_Device_Address(-1);
-#else
         datalink_get_my_address(&my_address);
-#endif
         npdu_encode_npdu_data(&npdu_data, true, MESSAGE_PRIORITY_NORMAL);
         pdu_len =
             npdu_encode_pdu(&Handler_Transmit_Buffer[0], &dest, &my_address,