[PATCH, libstdc++] Fix missing gthr-default.h issue on libstdc++ configure

[Kirill Yukhin]

>> Looks Ok. If David can test is successfully on AIX I can approve it.
>
> I was able to bootstrap successfully with the patch.

Checked in: http://gcc.gnu.org/ml/gcc-cvs/2012-10/msg00581.html

Thanks, K