Use steady_clock to implement condition_variable::wait_for

The C++ standard says that std::condition_variable::wait_for should be
implemented to be equivalent to:

  return wait_until(lock, chrono::steady_clock::now() + rel_time);

But the existing implementation uses chrono::system_clock. Now that
wait_until has potentially-different behaviour for chrono::steady_clock,
let's at least try to wait using the correct clock.

2018-08-01  Mike Crowe  <mac@mcrowe.com>

	* include/std/condition_variable (wait_for): Use steady_clock.

From-SVN: r263225

libstdc++-v3/ChangeLog

 2018-08-01  Mike Crowe  <mac@mcrowe.com>
 
+	* include/std/condition_variable (wait_for): Use steady_clock.
+
+2018-08-01  Mike Crowe  <mac@mcrowe.com>
+
 	* include/std/condition_variable (wait_until): Only report timeout
 	if we really have timed out when measured against the
 	caller-supplied clock.

libstdc++-v3/include/std/condition_variable

@@ -66,6 +66,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   class condition_variable
   {
     typedef chrono::system_clock	__clock_t;
+    typedef chrono::steady_clock	__steady_clock_t;
     typedef __gthread_cond_t		__native_type;
 
 #ifdef __GTHREAD_COND_INIT
@@ -144,11 +145,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       wait_for(unique_lock<mutex>& __lock,
 	       const chrono::duration<_Rep, _Period>& __rtime)
       {
-	using __dur = typename __clock_t::duration;
+	using __dur = typename __steady_clock_t::duration;
 	auto __reltime = chrono::duration_cast<__dur>(__rtime);
 	if (__reltime < __rtime)
 	  ++__reltime;
-	return wait_until(__lock, __clock_t::now() + __reltime);
+	return wait_until(__lock, __steady_clock_t::now() + __reltime);
       }
 
     template<typename _Rep, typename _Period, typename _Predicate>