nvcc exits compilation if the header dune/istl/basearray.hh (form DUNE
2.6) is included as it does not seem to understand the friend declaration
there (friend class for a struct).
```
/usr/include/dune/istl/basearray.hh:101:49: error: ‘typename Dune::Imp::base_array_unmanaged<B, A>::RealIterator’ names ‘template<class B, class A> template<class T> struct Dune::Imp::base_array_unmanaged<B, A>::RealIterator’, which is not a type
friend class RealIterator<const ValueType>;
^
```