configure.in: use ac_cv_func_vfork

* configure.in (ac_cv_func_vfork): ac_cv_func_vfork_works is
  overridden always.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47442 b2dd03c8-39d4-4d8f-98ff-823fe69b080e