GCC 4.8 provides 'nullptr' in C++11 mode. That capability is
detected at configuration time and stored in <config.h> and we need
it when targeting Dune 2.2.1 to avoid diagnostics of the form
[...]/dune/common/nullptr.hh:27:1:
error: expected ‘;’ after class definition
} nullptr = {}; // and whose name is nullptr
^
[...]/dune/common/nullptr.hh:27:1:
error: qualifiers can only be specified for objects and functions
[...]/dune/common/nullptr.hh:27:3:
error: expected unqualified-id before ‘nullptr’
} nullptr = {}; // and whose name is nullptr