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

[Pavel Chupin]

Please see attached patch (applicable after revert).
I've moved libgcc libstdc++ common configure thread header chunk into
separate gthr.m4.
Could you please try it on AIX?

Is it OK for trunk?
Is it OK for 4.7 after a week testing in trunk?
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