BUILD: epoll/threads: Add test on MAX_THREADS to avoid warnings when complied without threads

When HAProxy is complied without threads, gcc throws following warnings:

  src/ev_epoll.c:222:3: warning: array subscript is outside array bounds [-Warray-bounds]
  ...
  src/ev_epoll.c:199:11: warning: array subscript is outside array bounds [-Warray-bounds]
  ...

Of course, this is not a bug. In such case, tid is always equal to 0. But to
avoid the noise, a check on MAX_THREADS in "if (tid)" lines makes gcc happy.

This patch should be backported in 1.8 with the commit d9e7e36c ("BUG/MEDIUM:
epoll/threads: use one epoll_fd per thread").
diff --git a/src/ev_epoll.c b/src/ev_epoll.c
index 635b8a5..e5c0001 100644
--- a/src/ev_epoll.c
+++ b/src/ev_epoll.c
@@ -195,7 +195,7 @@
 	if (epoll_events == NULL)
 		goto fail_alloc;
 
-	if (tid) {
+	if (MAX_THREADS > 1 && tid) {
 		epoll_fd[tid] = epoll_create(global.maxsock + 1);
 		if (epoll_fd[tid] < 0)
 			goto fail_fd;
@@ -218,7 +218,7 @@
 
 static void deinit_epoll_per_thread()
 {
-	if (tid)
+	if (MAX_THREADS > 1 && tid)
 		close(epoll_fd[tid]);
 
 	free(epoll_events);