pure-ftpwho -c

i can't figure out the pure-ftpwho -w and pure-ftpwho -c commands for the
.cgi support i have read the documentation and can't find a solution for:

'-c': the program is called via a web server (CGI interface) . Output is a
full HTML page with the initial content-type header. This option is
automatically enabled if an environment variable called GATEWAY_INTERFACE is
found. This is the default if you can access the program from a CGI-enabled web
server (Apache, Roxen, Caudium, WN, ...) .

since i con't know where the variable called GATEWAY_INTERFACE should be
found, i don't know if i have to edit rc.conf or httpd.conf (using apache)
or pure-ftpd.conf ?
and if i should edit anything at all? so if anyone has ever done it pls help
;-)
thanks,
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