Enable assertions in the extra debug library built when
--enable-libstdcxx-debug is used. Replace some Debug Mode assertions
in src/c++11/futex.cc with __glibcxx_assert, because the library will
never be built with Debug Mode.

	* configure: Regenerate.
	* configure.ac: Add -D_GLIBCXX_ASSERTIONS to default DEBUG_FLAGS.
	* src/c++11/futex.cc: Use __glibcxx_assert instead of
	_GLIBCXX_DEBUG_ASSERT.

From-SVN: r263235