>> 	I'm still wondering how to identify the former case and the latter
>> 	case.  Candidates are as follows:
>> 	- Check kernel AF_INET6 support - looks unwise.  INET6 can be
>> 	  turned off.
>> 	- Check existence of certain file - like /usr/include/netinet6/ip6.h
>> 	- others?
>> 	I think the second option (certain file) is the way to go.  If you have
>> 	any other ideas, please let me know.
>
>You could patch the package so that it builds an ipv6 version if gnu
>configure finds the header file and sets HAVE_NETINET6_IP6_H
>(overridable by a configure option). That way, if the author accepts
>the patch, it will build out of the box on NetBSD-1.4.x, NetBSD-1.5,
>and maybe other systems, too.

	There are, of course, such cases exist.  But they are the lucky ones.
	- some software do not use autoconf at all.
	- some software require extra "--enable-ipv6" argument to configure.
	  (by default compiles to IPv4 only)
	- and more wacky cases as much as you can imagine :-)

itojun