PR libstdc++/60421
	* include/std/thread (this_thread::sleep_for): Retry on EINTR.
	(this_thread::sleep_until): Retry if time not reached.
	* src/c++11/thread.cc (__sleep_for): Retry on EINTR.
	* testsuite/30_threads/this_thread/60421.cc: Test interruption and
	non-steady clocks.

From-SVN: r230183