* configure.in: don't test xti.h here.

* ext/socket/extconf.rb: test xti.h here.

  Originally, xti.h is included for IRIX [ruby-core:14447].



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39262 b2dd03c8-39d4-4d8f-98ff-823fe69b080e