Fix compiler warnings on NetBSD
configure: check if pkg-config is available

This is needed with some shells to prevent "pkg-config: not found"
errors from being displayed.
ntp: print warning 10 years before supported time ends
Ignore measurements around leap second

When current time is within 5 seconds of a leap second, don't accumulate
new samples or update the leap second status to increase the chances of
getting through safely.