[Dune] Backporting recent nullptr fix?

[Christian Engwer]

Hi Oli,

> Something like this is definitely needed, because the current
> dune-common 2.1
> cannot be used with g++ 4.6.  On the other hand that commit is an API change
> because it moves nullptr_t around.  I am not sure whether the fix and the
> API change can be separated?

the alternative would be to pull std::nullptr_t into the global
namespace. Actually it will not make much difference, because nowhere
in Dune the type was used. So I don't actually consider it an API
change, but merely a change in the implementation. We added nullptr
because we needed the singleton and that es all that was used.

Christian