Stop finding old SuperLU versions.

Previously CMake pretended to have found a valid SuperLU version for DUNE even if the version was too old (i.e. < 4). People having such an old versions only noticed this via later compile errors of binaries using SuperLU. That is far too late.

With this patch we introduce a compile check into the CMake module that fails for old unsupported versions. In that case CMake will not find SuperLU.