[Patch] libstdc++/19664

[Paolo Carlini]

Benjamin Kosnik wrote:

>I thought we were going to apply this to namespaces.
>  
>
Or you want to 'suspend' completely this PR until visibility attributes
can be applied to namespaces?!? In that case we give up completely with
the hope of having something functional in a reasonable time-frame, I'm
afraid, and nobody discussed this plan in public, 'til today... I like
the idea of visibility on namespaces, don't get me wrong, but we should
make sure it's actually feasible and someone is going to work on it...

Paolo.