PR libstdc++/61841
	* include/std/thread (thread::_M_start_thread): Declare new overload.
	(thread::thread<_Callable, _Args...>): Call new overload with an
	explicit reference to pthread_create.
	* src/c++11/thread.cc (thread::_M_start_thread): Add new overload.
	* config/abi/pre/gnu.ver: Export new function.

From-SVN: r213922