[master] Hnadle dhcpv6 being disabled better

The previous patches didn't handle dhvpv6 being disabled well.
    This change moves the #ifdefs a bit to handle it.

relay/dhcrelay.c

@@ -183,9 +183,11 @@ char *progname;
  * \return Nothing
  */
 static const char use_noarg[] = "No argument for command: %s";
+#ifdef DHCPv6
 static const char use_badproto[] = "Protocol already set, %s inappropriate";
 static const char use_v4command[] = "Command not used for DHCPv6: %s";
 static const char use_v6command[] = "Command not used for DHCPv4: %s";
+#endif
 
 static void
 usage(const char *sfmt, const char *sarg) {