I'm still wondering how IPv6-ready packages should be handled.

	To summarize, there was rough consensus on nuke USE_INET6 (an explicit
	flag to turn on IPv6 support) and go to HAVE_INET6 (an automatically
	defined flag to turn on IPv6 support).
	starting from:
	http://mail-index.netbsd.org/tech-pkg/2000/01/08/0000.html

	My goals are like this.  For dual stack capable packages (like
	fetchmail):
	- pre-1.4x users need to build packages.  for them IPv6 support should
	  be disabled.
	- for post-1.4x (after IPv6 merge), IPv6 support should always be
	  enabled.
	for packages which may break IPv4 part:
	- prepare foo and foo6 separately (like apache/apache6)
	- on pre-1.4x, foo6 will not be built (BROKEN)
	- on post-1.4x, foo6 can be built fine

	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.

	other thing: is HAVE_INET6 the right name?

itojun