zabbix autoconf script does not use the system compiler by default

The zabbix autoconf script search, by default for the ``gcc'' compiler command. This is not standard.
Any standard-compliant specification of ``make'', MUST define the CC environment variable if not previously set with a default value (could be ``cc'' or ``c99'') and if the CC environment variable is exported before configure the exported value must be used.

This actually affect all the platforms that does not use gcc as a system compiler, causing the user to force the setting of the CC environment variable to override what the zabbix configure script want to do.

It's just a minor annoyance, but respecting the standards is always good