Both 1.8.15 and 2.0.2 appear to reference a (bash?) function called 'use' during the install process. Unfortunately, Gentoo has already defined a function with that name that is part of its package management software (portage). A bug within Gentoo has been opened by the QA Team against Zabbix 1.8.15 and 2.0.2 to block stabilization of those and future versions of Zabbix until the issue has been resolved.

Please see https://bugs.gentoo.org/show_bug.cgi?id=433090 for more info.