zabbix does not compile if operating system cannot be determined

As the subject says, Zabbix fails to compile on an unknown platform ("unknown" in the sense of src/libs/zbxsysinfo/Makefile.am - "aix", "freebsd", ..., "solaris", "unknown").

One example of such a platform is kFreeBSD (https://wiki.debian.org/Debian_GNU/kFreeBSD) and was reported to us by Dmitry Smirnov, the maintainer of Zabbix package on Debian.