Some shadow fixes.

[Martin v. Loewis]

> Portable standard-conforming C++ code cannot depend 
> on C9X features, and users need to be able to build in a 
> standard-conforming and -enforcing environment.

In GCC, as well as glibc, providing the extensions is the default; the
user has to ask explicitly for a pedantic environment.

I don't know whether this is relevant to this discussion, but I'd
prefer if libstdc++ followed this tradition.

Regards,
Martin