As we have bumped our compiler requirements to GCC 4.4+, we can now
require the compiler to support C++11.

This patch updates both the autotools and the CMake tests to produce an
error if the compiler doesn't support either -std=c++11 or -std=c++0x.

It also renames the tests from ..0X to ..11 and renames the configure
options from --disable-gxx0xcheck to --disable-cxx11check and removes a
couple of redundant calls to the autotools macro.

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.