Fix to detect Thread Execution Scheduling support correctly

If `_POSIX_THREAD_PRIORITY_SCHEDULING == 0`, it must be checked
with sysconf(_SC_THREAD_PRIORITY_SCHEDULING) at runtime.

Signed-off-by: OBATA Akio <obache@wizdas.com>
(cherry picked from commit d61a821ad0)

src/kernel/qthread_unix.cpp

@@ -314,7 +314,10 @@ void QThread::start(Priority priority)
     pthread_attr_init(&attr);
     pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
 
-#if !defined(Q_OS_OPENBSD) && defined(_POSIX_THREAD_PRIORITY_SCHEDULING) && (_POSIX_THREAD_PRIORITY_SCHEDULING-0 >= 0)
+#if defined(_POSIX_THREAD_PRIORITY_SCHEDULING) && (_POSIX_THREAD_PRIORITY_SCHEDULING-0 >= 0)
+#if _POSIX_THREAD_PRIORITY_SCHEDULING == 0 && defined _SC_THREAD_PRIORITY_SCHEDULING
+    if (sysconf(_SC_THREAD_PRIORITY_SCHEDULING) > 0)
+#endif
     switch (priority) {
     case InheritPriority:
 	{