Disable some concept checks in C++11

[Jonathan Wakely]

On 27 February 2017 at 20:11, François Dumont wrote:
> Hi
>
>
>     I had some problems when testing pretty printers while having activated
> concept checks. I noticed that std::deque had already the
> _SGIAssignableConcept check disable when using C++11 so I propose to
> generalize to all usages of this concept check.

The use in <ext/slist> seems to still be valid, that type wasn't
updated for C++11 and move semantics, so the original concept checks
still apply, don't they?

I think all the other changes to remove Assignable in C++11 are
correct, and I see no problem making this change now, because it only
relaxes some checks that are not used by most people and not used in
the default configuration.