[v3] Fix second half of libstdc++/6745

[Gabriel Dos Reis]

Benjamin Kosnik <bkoz@redhat.com> writes:

| >A potentially much simpler solution occured to me.  Could we declare that
| >Paolo's original patch (the one that started this thread) only applies to
| >cin?  I.e., make the condition this:
| >    if (__sbin == cin->rdbuf() && isatty(0))
| 
| Hey y'all. I think a better way to think about this is that the
| streambuf is unbuffered. That way you don't have to deal with the isatty
| issue. (For unbuffered streams, _M_buf_size_opt == 0)
| 
| If you do the checks based on that I think it will be cleaner. 

I would favor that approach, though I would have to rethink about it.

-- Gaby