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

[Kirill Yukhin]

Reverted.
Trunk: http://gcc.gnu.org/ml/gcc-cvs/2012-10/msg00516.html
4.7: http://gcc.gnu.org/ml/gcc-cvs/2012-10/msg00517.html

K