demand support is now a compile-time option

Obviously, it requires some wiring to know if there is new activity
on a not-yet established PPP interface with the default route
already set.

I don't think any lwIP user will ever need that, all should know
when to bring the link up and down.

src/netif/ppp/upap.c

@@ -105,8 +105,10 @@ struct protent pap_protent = {
     pap_option_list,
     NULL,
 #endif /* PPP_OPTIONS */
+#if DEMAND_SUPPORT
     NULL,
     NULL
+#endif /* DEMAND_SUPPORT */
 };
 
 upap_state upap[NUM_PPP];		/* UPAP state; one for each unit */