Add libc++ version detection.

git-svn-id: http://svn.boost.org/svn/boost/trunk@83925 b8fc166d-592f-0410-95f2-cb63ce0dd405
Spirit: making sure destructor of temporary stream is called in the right places

git-svn-id: http://svn.boost.org/svn/boost/trunk@83926 b8fc166d-592f-0410-95f2-cb63ce0dd405