use POSIX semaphores in shmtx instead of sched_yield()

number of spinlock spins are increased twice

auto/os/freebsd

@@ -49,6 +49,15 @@ if [ $osreldate -gt 502103 ]; then
     have=NGX_HAVE_AIO_SENDFILE . auto/have
 fi
 
+# POSIX semaphores
+# http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/127545
+
+if [ $osreldate -ge 701106 ]; then
+    echo " + POSIX semaphores should work"
+else
+    have=NGX_HAVE_POSIX_SEM . auto/nohave
+fi
+
 
 # kqueue