[v3] Fix libstdc++/54577

[Paolo Carlini]

On 05/14/2013 02:40 PM, Paolo Carlini wrote:
> Then I suppose that the correct way to move forward to C++11 the 
> ext/pointer.h stuff would be adding a pointer_traits specialization 
> for those pointer-like types, which would also wrap the cast 
> operations in pointer_to. Then, in __normal_iterator::_M_const_cast 
> use pointer_traits.
... and even more interestingly, looks like Jonathan already did most of 
this, at the end of ext/pointer.h. Oh my ;)

Paolo.