thread.c: remove unnecessary sentence

* thread.c (thread_start): initialize method is not concerned with
  Thread.start and Thread.fork.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39485 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
configure.in: find Setup file automatically

* configure.in (setup): find Setup file from target_os 1. by
  suffix (e.g. Setup.nacl, Setup.atheos), 2. by "platform"
  option (e.g. Setup.nt, Setup.emx), and 3. default Setup.  And
  Setup.dj had been removed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39486 b2dd03c8-39d4-4d8f-98ff-823fe69b080e