RT#89635 Configure on 5.6 uses sort +N, which isn't supported by modern sort(1)

The two instances of 'sort -n +1' in Configure on perl 5.6 need to be
replaced with the portable equivalent used since perl 5.7.3:

  ($sort -n -k 2 2>/dev/null || $sort -n +1)

This works on both old and new sorts.