socket/extconf.rb: reduce duplicated code
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49539 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
socket/extconf.rb: reduce duplicated code
value first, then check OS-provided functions.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49539 b2dd03c8-39d4-4d8f-98ff-823fe69b080e