net-mgmt/zabbix34-server: Fix build with non-default LOCALBASE

Upstream's configure script assumes that libevent will be in
/usr/local rather than using pkg-config. This causes builds on
systems with non-default LOCALBASE settings to fail. We are able to
work around this by specifying the correct path in the argument to
--with-libevent.

PR: 224304
Submitted by: Erick Turnquist <jhujhiti@adjectivism.org>
Approved by: pg@pakhom.spb.ru (maintainer timeout, 3 months)