[cmake] Don't check for C++17 features which we require anyway

Set the following variables always to 1:
 DUNE_HAVE_CXX_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
 DUNE_HAVE_CXX_OPTIONAL
 DUNE_HAVE_CXX_VARIANT
 DUNE_SUPPORTS_CXX_THROW_IN_CONSTEXPR
 DUNE_HAVE_C_ALIGNED_ALLOC