apply the two caching patches proposed by Christoph Grüninger,
at least for me they worked flawless and showed some considerable configure speedup.

[[Imported from SVN: r6539]]

* update configure check
* move nullptr_t to namespace Dune
* import nullptr_t from namespace std (according to updated proposal)

[[Imported from SVN: r6492]]

[[Imported from SVN: r6305]]