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/writefile/main.c

@@ -279,7 +279,7 @@ int main(
                     fclose(pFile);
                 } else
                     End_Of_File_Detected = true;
-                printf("\rSending %d bytes", (fileStartPosition + len));
+                printf("\rSending %zu bytes", (fileStartPosition + len));
                 invoke_id =
                     Send_Atomic_Write_File_Stream
                     (Target_Device_Object_Instance,