[v3] fix PR 53978

[Jonathan Wakely]

On 16 July 2012 21:44, Jonathan Wakely wrote:
>         PR libstdc++/53978
>         * include/bits/alloc_traits.h (allocator_traits): Define static
>         constants.
>         * include/bits/ptr_traits.h (pointer_traits): Likewise.
>         * include/ext/alloc_traits.h (__allocator_always_compares_equal):
>         Likewise.
>
> Tested x86_64-linux, committed to trunk.

Also committed to 4.7 (without copyright year ranges)