* include/bits/c++config (_GLIBCXX_USE_WEAK_REF): New.
	(_GLIBCXX_TXN_SAFE, _GLIBCXX_TXN_SAFE_DYN): Use	_GLIBCXX_USE_WEAK_REF
	and move after its definition.
	* config/os/aix/os_defines.h (_GLIBCXX_USE_WEAK_REF): Override.
	* src/c++11/cow-stdexcept.cc: Use _GLIBCXX_USE_WEAK_REF instead of
	__GXX_WEAK__, and only provide transactional clones if
	_GLIBCXX_USE_WEAK_REF is true.  Don't provide stubs of libitm
	functions.

From-SVN: r232539