List::Util::XS can be used as a dependency to ensure List::Util was
installed using a C compiler and that the XS version is installed.
During installation $List::Util::XS::VERSION will be set to
undef if the XS was not compiled.
Starting with release 1.23_03, Scalar-List-Util is <B>alwaysB> using
the XS implementation, but for backwards compatibility, we still
ship the List::Util::XS module which just loads List::Util.