use autoconf results to conditionally compile timersub

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89372 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/channel.h

@@ -1357,7 +1357,7 @@ static inline int ast_fdisset(struct pollfd *pfds, int fd, int max, int *start)
 	return 0;
 }
 
-#ifdef SOLARIS
+#ifndef HAVE_TIMERSUB
 static inline void timersub(struct timeval *tvend, struct timeval *tvstart, struct timeval *tvdiff)
 {
 	tvdiff->tv_sec = tvend->tv_sec - tvstart->tv_sec;