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/include/datalink.h

@@ -81,7 +81,11 @@
 #endif
 #define datalink_cleanup bip_cleanup
 #define datalink_get_broadcast_address bip_get_broadcast_address
+#if BAC_ROUTING
+#define datalink_get_my_address routed_get_my_address
+#else
 #define datalink_get_my_address bip_get_my_address
+#endif
 
 #else /* Ie, BACDL_ALL */
 #include "npdu.h"