socket/extconf.rb: check struct in_addr6

* ext/socket/extconf.rb: check struct in_addr6, which is defined in
  VC6 instead of in6_addr.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40192 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
socket/extconf.rb: add -D

* ext/socket/extconf.rb: $defs needs -D or -U.  nothing is added
  otherwize.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40193 b2dd03c8-39d4-4d8f-98ff-823fe69b080e