Add an option to disable IPv6 support

In jail environment, because fping 4.0 comes
with combined IPv4/IPv6 support, you need to enable
IPv6 for the jail even if you will not use IPv6.

This patch adds an IPV6 option, enabled by default,
but who can be turn off to build fping without IPv6
support and used it inside a jail without IPv6.

PR: 229903
Submitted by: Andrew <andrew.hotlab@hotmail.com>
Reported by: Stefan Witzel <stefan.witzel@zvw.uni-goettingen.de>
Approved by: <jharris@widomaker.com> (maintainer)