Update checks for AD-types.

Update checks for AD-types.

At some point these stopped working and something like

#include <deal.II/lac/vector.h>
#include <deal.II/lac/vector.templates.h>

#include <adolc/adouble.h>

int
main()
{
  using namespace dealii;

  Vector<adouble> vec;
}

managed to compile (but compiling Vector<adouble>::l2_norm() fails).

In practice we don't support any type here which we cannot call std::abs() on,
so lets just switch to a simpler check for these types.