Beeing glad I'm not the only one who argues about the unportability of s6_add32, here is what the cups authors think about it.

/*
 * Oh, the wonderful world of IPv6 compatibility.  Apparently some
 * implementations expose the (more logical) 32-bit address parts
 * to everyone, while others only expose it to kernel code...  To
 * make supporting IPv6 even easier, each vendor chose different
 * core structure and union names, so the same defines or code
 * can't be used on all platforms.
 *
 * The following will likely need tweaking on new platforms that
 * support IPv6 - the "s6_addr32" define maps to the 32-bit integer
 * array in the in6_addr union, which is named differently on various
 * platforms.
 */
 
#if defined(AF_INET6) && !defined(s6_addr32)
#  if defined(__sun)
#    define s6_addr32	_S6_un._S6_u32
#  elif defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) || defined(__APPLE__)
#    define s6_addr32	__u6_addr.__u6_addr32
#  elif defined(__osf__)
#    define s6_addr32	s6_un.sa6_laddr
#  elif defined(WIN32)
/*
 * Windows only defines byte and 16-bit word members of the union and
 * requires special casing of all raw address code...
 */
#    define s6_addr32	error_need_win32_specific_code
#  endif /* __sun */
#endif /* AF_INET6 && !s6_addr32 */

source: cups