memory, [...]: Use mutex to make _Sp_counted_base::add_ref_lock() thread-safe.

	* include/tr1/memory, include/tr1/boost_shared_ptr.h: Use mutex
	to make _Sp_counted_base::add_ref_lock() thread-safe. Check whether
	to destroy resources by testing for equality, not inequality. Add
	empty critical sections to solve memory visibility issues.
	* testsuite/tr1/2_general_utilities/memory/
	shared_ptr/cons/auto_ptr_neg.cc: Use dg-excess-errors instead of
	explicitly listing line numbers which need to be kept in sync.
	* testsuite/tr1/2_general_utilities/memory/
	shared_ptr/assign/auto_ptr_neg.cc: Same.
	* testsuite/tr1/2_general_utilities/memory/
	shared_ptr/assign/auto_ptr_rvalue_neg.cc: Same.
	* testsuite/tr1/2_general_utilities/memory/
	shared_ptr/cons/weak_ptr_expired.cc: Make XFAIL for consistency when
	-fno-exceptions.
	* testsuite/tr1/2_general_utilities/memory/
	enable_shared_from_this/not_shared.cc: Add explanatory comments.
	* testsuite/tr1/2_general_utilities/memory/
	enable_shared_from_this/not_shared2.cc: Same.
	* testsuite/tr1/2_general_utilities/memory/
	enable_shared_from_this/not_shared3.cc: Same.

From-SVN: r97620