[c++11] add support for C++11 constexpr

As decided on the developer meeting in Aachen, we support (but do not
rely on) the C++11 keyword constexpr. This patch adds an m4-check for
constexpr and a header (cxx11.hh) defining a macro DUNE_CONSTEXPR either
as constexpr or empty (depending on the compiler support for it). It is
then used in FieldVector and FieldMatrix on size, rows, and cols.