• Detect presence of sockaddr_in6 and ipv6_mreq; conditionally build
  pack/unpack functions on this
  
• Back-compatibility improvements for older perls, back as far as
  5.6.1 (thanks Zefram)
  
• Fix for picky compilers or platforms on which size_t doesn't
  printf() correctly by %d
  
• Suppress some harmless compile-time warnings about unused variables