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

[Kirill Yukhin]

Hi guys,

>> Is it ok for release it into trunk and 4.7?
>
> Yes, please do so.

Checked into trunk: http://gcc.gnu.org/ml/gcc-cvs/2012-10/msg00419.html
and 4.7: http://gcc.gnu.org/ml/gcc-cvs/2012-10/msg00431.html

Thanks, K