configure option `--disable-fastthread', to choose the original,
  pure ruby version of the "thread" library instead of the new,
  much faster implementation in ext/thread.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8@11698 b2dd03c8-39d4-4d8f-98ff-823fe69b080e