[Dune] Backporting recent nullptr fix?

[Oliver Sander]

Dear Dune!
I pondering whether to backport Christian's and Steffen'S recent nullptr
fix to dune-common 2.1.

------------------------------------------------------------------------
r6492 | christi | 2011-09-06 19:04:07 +0200 (Di, 06. Sep 2011) | 4 Zeilen

make nullptr play nicely with g++4.6
* update configure check
* move nullptr_t to namespace Dune
* import nullptr_t from namespace std (according to updated proposal)

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?

Opinions?

cheers,
Oliver