zebra: fix rtadv detection

The GLIBC macro tests were just plain wrong. Glibc 2.1 is ancient
and the support should be detected via configure.ac test, not
macro test. Build with HAVE_RTADV but !RTADV is broke after vrf,
so for now, just define RTADV always if HAVE_RTADV is there.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
diff --git a/zebra/rib.h b/zebra/rib.h
index 8328f23..2af90db 100644
--- a/zebra/rib.h
+++ b/zebra/rib.h
@@ -333,12 +333,8 @@
 };
 
 /* Router advertisement feature. */
-#ifndef RTADV
-#if (defined(LINUX_IPV6) && (defined(__GLIBC__) && __GLIBC__ >= 2 && __GLIBC_MINOR__ >= 1)) || defined(KAME)
-  #ifdef HAVE_RTADV
-    #define RTADV
-  #endif
-#endif
+#if !defined(RTADV) && defined(LINUX_IPV6) && defined(HAVE_RTADV)
+#  define RTADV
 #endif
 
 #if defined (HAVE_IPV6) && defined (RTADV)