Making AVG_NUM configurable by compiler define (Submitted by Lars)

A minor change which makes the AVG_NUM configurable by compiler define.
Partly Closed #956 and Closes #1166
Fixing bug in average calculation (Submitted by Lars)

The more important change: A fix for a bug which caused the check_ntp_time
and check_ntp_time check to always take the first response from ntp and not
the avarage since "j" is not increased anywhere. "i" should be used there
instead.

Partly Closes #956 and Closes #1166
Closes: #1089