re PR libstdc++/52680 (std::this_thread::sleep_for #ifdef'd out by _GLIBCXX_USE_NANOSLEEP)

	PR libstdc++/52680
	* acinclude.m4 (GLIBCXX_ENABLE_LIBSTDCXX_TIME): Check for usleep and
	sleep if nanosleep is not available. Bump libtool revision.
	* config.h.in: Regenerate.
	* configure: Likewise.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.18): Add __sleep_for.
	* include/std/thread (this_thread::__sleep_for): Add.
	(this_thread::yield, this_thread::sleep_until, this_thread::sleep_for):
	Declare unconditionally.
	* src/c++11/thread.cc (this_thread::__sleep_for): Define.
	* testsuite/lib/libstdc++.exp (check_v3_target_nanosleep): Rename to
	check_v3_target_sleep.
	* testsuite/lib/dg-options.exp (dg-require-nanosleep): Rename to
	dg-require-sleep.
	* testsuite/30_threads/condition_variable_any/53830.cc: Update.
	* testsuite/30_threads/this_thread/2.cc: Likewise.
	* testsuite/30_threads/this_thread/3.cc: Likewise.
	* testsuite/30_threads/this_thread/4.cc: Likewise.
	* testsuite/30_threads/async/54297.cc: Likewise.

From-SVN: r193769