* ext/socket/extconf.rb: Make default_ipv6 true for Cygwin.
  Cygwin supports IPv6 since Cygwin 1.7.1 (2009-12).
  http://cygwin.com/ml/cygwin-announce/2009-12/msg00027.html



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40570 b2dd03c8-39d4-4d8f-98ff-823fe69b080e