from terminate_spec.rb, as suggested here:
https://github.com/ruby/ruby/commit/c7220bd3dea0a113b81f267cc3b656da7d7d7abb#commitcomment-31068714

to guard other inclusions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65392 b2dd03c8-39d4-4d8f-98ff-823fe69b080e