shared_mutex (shared_timed_mutex): Only use pthread_rwlock_t when the POSIX Timeouts option is supported.

	* include/std/shared_mutex (shared_timed_mutex): Only use
	pthread_rwlock_t when the POSIX Timeouts option is supported.
	* testsuite/30_threads/shared_lock/cons/5.cc: Remove
	dg-require-gthreads-timed.
	* testsuite/30_threads/shared_lock/cons/6.cc: Likewise.
	* testsuite/30_threads/shared_lock/locking/3.cc: Likewise.
	* testsuite/30_threads/shared_lock/locking/4.cc: Likewise.

From-SVN: r221971