socket: fix build error and warnings on mingw

* ext/socket/extconf.rb: check for if_nametoindex() for
  i686-w64-mingw32, and check for declarations of if_indextoname() and
  if_nametoindex().
* ext/socket/ifaddr.c (ifaddr_ifindex): not-implement unless
  if_nametoindex() is available.
* ext/socket/rubysocket.h: declare if_indextoname() and
  if_nametoindex() if available but not declared.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40738 b2dd03c8-39d4-4d8f-98ff-823fe69b080e