Daily chksetuid script - how to ignore certain dirs/filesystems?

[Pat Maddox]

I have a backup script that runs every night, backing up everything to
a backup drive.  When the security script runs, it finds a bunch of
setuid files at /backup - I'd like to ignore those files, so I don't
have to wade through them every day.  I also back up to a remote
server and it results in the same thing.  How can I make it skip over
the backup dir, or at least ignore it in the output?  The cron file in
question is /etc/periodic/security/100.chksetuid

Pat