* ext/curses/extconf.rb: Test linkability of curses_version at first.

* ext/socket/extconf.rb: Test the behavior of fd passing with MSG_PEEK
  only if recvmsg(), msg_control member, AF_UNIX and SCM_RIGHTS are
  available.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40473 b2dd03c8-39d4-4d8f-98ff-823fe69b080e