Bump WARNS to 6:

Bump WARNS to 6:
- Remove redundant nested extern declarations;
- Constify some char *'s, introducing new variables where existing
  variables were being used more than once for different purposes;
- Adjust the signedness of some integer variables to reflect their
  usage in comparisons;
- Use a struct timeval instead of a pair of integer variables to
  record the 'best' time.