Ruby

extmk.rb: fix with-ext condition

• ext/extmk.rb: configure intersection of with-ext and not
  without-ext, as withouts is no longer true by default if
  with-ext option is given. [ruby-dev:49108] [Bug #11280]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50959 b2dd03c8-39d4-4d8f-98ff-823fe69b080e