(Hopefully) better fix for Windows warning on redefined TAILQ_EMPTY;

the previous attempt broke build on Debian4.

extra/libevent/event-internal.h

@@ -69,14 +69,17 @@ struct event_base {
 };
 
 /* Internal use only: Functions that might be missing from <sys/queue.h> */
-#ifndef HAVE_TAILQFOREACH
 /* These following macros are copied from BSD sys/queue.h
    Copyright (c) 1991, 1993, The Regents of the University of California.
    All rights reserved.
 */
+#ifndef TAILQ_EMPTY
+#define	TAILQ_EMPTY(head)		((head)->tqh_first == NULL)
 #define	TAILQ_FIRST(head)		((head)->tqh_first)
 #define	TAILQ_END(head)			NULL
 #define	TAILQ_NEXT(elm, field)		((elm)->field.tqe_next)
+#endif /* TAILQ_EMPTY */
+#ifndef HAVE_TAILQFOREACH
 #define TAILQ_FOREACH(var, head, field)					\
 	for((var) = TAILQ_FIRST(head);					\
 	    (var) != TAILQ_END(head);					\