Avoid creating an unnecessary thread pool

If the thread_pool does not already exist, there is no point in
creating a thread pool just to join the threads in it.

This is an alternative fix for ruby#440.

lib/rake/application.rb

@@ -124,7 +124,7 @@ def run_with_threads
 
       yield
 
-      thread_pool.join
+      thread_pool.join if defined?(@thread_pool)
       if options.job_stats
         stats = thread_pool.statistics
         puts "Maximum active threads: #{stats[:max_active_threads]} + main"