It would be nice to have IPv6 support enabled, as it's already integrated. Just add --enable-ipv6 to %configure, that's it :-)
My first question would be, why doesn't upstream have it on by default?
I guess there are two reasons: 1) they don't have a proper autoconf check that would enable it only if the compiling box has IPv6 enabled (these come up now and then), and 2) as xchat is bound to be built on older and weirder architectures too, defaulting to on might break compiling for these if1) is not implemented . After all, the API has changed a lot, and IPv6 isn't exactly mainstream yet... I don't see a big hurry with this though, but other apps are getting enabled..
IPv6 is now enabled in rawhide build xchat-1.8.7-4