zebra: Cleanup RTADV define

The RTADV define was not being set correctly or consistently.
Make the code consistent with our HAVE_IPV6 define.

If the user wants to explicitly turn it off then they should
run --disable-rtadv from the configure cli

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/zebra/interface.c b/zebra/interface.c
index 162439f..411712d 100644
--- a/zebra/interface.c
+++ b/zebra/interface.c
@@ -42,11 +42,11 @@
 #include "zebra/debug.h"
 #include "zebra/irdp.h"
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
 /* Order is intentional.  Matches RFC4191.  This array is also used for
    command matching, so only modify with care. */
 const char *rtadv_pref_strs[] = { "medium", "high", "INVALID", "low", 0 };
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
 /* Called when new interface is added. */
 static int
@@ -59,7 +59,7 @@
   zebra_if->multicast = IF_ZEBRA_MULTICAST_UNSPEC;
   zebra_if->shutdown = IF_ZEBRA_SHUTDOWN_OFF;
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
   {
     /* Set default router advertise values. */
     struct rtadvconf *rtadv;
@@ -85,7 +85,7 @@
 
     rtadv->AdvPrefixList = list_new ();
   }    
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
   /* Initialize installed address chains tree. */
   zebra_if->ipv4_subnets = route_table_init ();
@@ -638,7 +638,7 @@
   vty_out (vty, "%s", VTY_NEWLINE);
 }
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
 /* Dump interface ND information to vty. */
 static void
 nd_dump_vty (struct vty *vty, struct interface *ifp)
@@ -699,7 +699,7 @@
 		 VTY_NEWLINE);
     }
 }
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
 /* Interface's information print out to vty interface. */
 static void
@@ -806,9 +806,9 @@
 	connected_dump_vty (vty, connected);
     }
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
   nd_dump_vty (vty, ifp);
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
 #ifdef HAVE_PROC_NET_DEV
   /* Statistics print out using proc file system. */
@@ -1782,9 +1782,9 @@
 		     VTY_NEWLINE);
 	}
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
       rtadv_config_write (vty, ifp);
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
 #ifdef HAVE_IRDP
       irdp_config_write (vty, ifp);