Actually, I think I misspoke my thoughts in my last post.  What I think
would be best for USE_INET6 is:

- USE_INET6 would not exist at all.

- A conditional flag, HAS_INET6, would be detected by bsd.pkg.mk to compile
  in IPv6 support, if the pkg cannot detect this during compile.

- Only pkgs which have _incompatible_ IPv4 and IPv6 functionality, or those
  which cannot detect IPv6 at runtime, should be compilable with and without
  IPv6, as _separate_ pkgs (progname and progname6).

Binary pkgs would therefore contain IPv6 support in all possible pkgs.

-- 
-- Todd Vierling (tv@pobox.com)