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/rib.h b/zebra/rib.h
index 1d3c15c..ffe7e2f 100644
--- a/zebra/rib.h
+++ b/zebra/rib.h
@@ -304,12 +304,8 @@
   vrf_id_t vrf_id;
 };
 
-/* Router advertisement feature. */
-#if !defined(RTADV) && defined(LINUX_IPV6) && defined(HAVE_RTADV)
-#  define RTADV
-#endif
 
-#if defined (RTADV)
+#if defined (HAVE_RTADV)
 /* Structure which hold status of router advertisement. */
 struct rtadv
 {
@@ -321,7 +317,7 @@
   struct thread *ra_read;
   struct thread *ra_timer;
 };
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
 #ifdef HAVE_NETLINK
 /* Socket interface to kernel */
@@ -370,9 +366,9 @@
   struct list *rid_lo_sorted_list;
   struct prefix rid_user_assigned;
 
-#if defined (RTADV)
+#if defined (HAVE_RTADV)
   struct rtadv rtadv;
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 };
 
 /*