Remove g++ signed/unsigned comparison warning in generated scanner
    
    Compiling a flex generated scanner with g++ with all warnings turned on (-Wall)
    produces a warning due to "comparison of integer expressions of different
    signedness". This change removes the warning by using the same signedness.
    
    This bug was a regression inherited from upstream flex during the rebase to
    flex-2.6.1. The same bug was fixed previously (albeit differently) by commit
    a3088c99e8.