net-mgmt/mk-livestatus: Unbreak Nagios 3.x support

Fix the NAGIOS4 option: The --without-nagios4 configure arg seems
to be broken in 1.2.8p20 and actually also enables Nagios 4.x
support. Switch to just passing --with-nagios4 if NAGIOS4=on for
the time being.

$ mk-livestatus-1.2.8p20> ./configure --without-nagios4
$ mk-livestatus-1.2.8p20> grep NAGIOS Makefile
CPPFLAGS = -g -O2 -DNAGIOS4

$ mk-livestatus-1.2.8p20> ./configure --with-nagios4
$ mk-livestatus-1.2.8p20> grep CPPFLAGS Makefile
CPPFLAGS = -g -O2 -DNAGIOS4

$ mk-livestatus-1.2.8p20> ./configure
$ mk-livestatus-1.2.8p20> grep CPPFLAGS Makefile
CPPFLAGS =

This makes sure that mk-livestatus is built with Nagios 3.x support
again by default.

PR: 232716
Submitted by: Leonid Vasiliev <leo9641@yandex.ru>