The configure script has both generic issues (fixed in later autoconf versions than 2.13), and a package-specific problem in the net-snmp detection code. I'm filing this bug for tracking purposes because upstream does not appear to have a bug tracker.
Created attachment 1937749 [details] ifstat-configure-snmp-c99.patch
Sent a note to the upstream author, too.
(In reply to Florian Weimer from comment #2) > Sent a note to the upstream author, too. Unfortunately, that bounced …