the dune/common/{unused,deprecated}.hh headers do not get implicitly
included by the OPM build system. Since the test for valgrind is has
been added recently, this file produced a compiler error if
HAVE_VALGRIND is 1.
use 'const' instead to make old compilers happy. the disadvantage is
that you cannot use the normal methods for template specialization,
but this currently should not be a major drawback.
