make this default to off.
additionally, check that the found superlu version
is compatible with the dune-istl version in use.
in particular superlu5 is not compatible with dune-istl 2.4
I ran into problems (syntax errors in if statements) if that module
bailed out before find_package_handle_standard_args() was
invoked. Since OPM/ewoms#54 this happened e.g. on systems where no
BLAS library is installed.
