thread_pthread.c: fix for dynamic PTHREAD_STACK_MIN

* thread_pthread.c (rb_thread_create_timer_thread): fix for platforms
  where PTHREAD_STACK_MIN is a dynamic value and not a compile-time
  constant.  [ruby-dev:47911] [Bug #9436]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44675 b2dd03c8-39d4-4d8f-98ff-823fe69b080e