Backward headers need fixing re. namespace

[Paolo Carlini]

Philipp Thomas wrote:

>Seems like some things where moved to namespace __gnu_cxx but the backward
>headers weren't kept up-to-date. Here is a patch to fix that.
>
>OK for mainline and branch?
>
Hi Philip. 

I can approve commits in this area. Therefore Ok for mainline. Please allow a couple of days before moving the patch to the branch.

Thanks!

Paolo.