Fix race condition in thread pool.

This was failing in JRuby, causing Travis to complain.

Author: jimweirich

lib/rake/thread_pool.rb

@@ -109,13 +109,19 @@ def process_queue_item      #:nodoc:
       false
     end
 
+    def safe_thread_count
+      @threads_mon.synchronize do
+        @threads.count
+      end
+    end
+
     def start_thread # :nodoc:
       @threads_mon.synchronize do
         next unless @threads.count < @max_active_threads
 
         t = Thread.new do
           begin
-            while @threads.count <= @max_active_threads
+            while safe_thread_count <= @max_active_threads
               break unless process_queue_item
             end
           ensure
@@ -126,6 +132,7 @@ def start_thread # :nodoc:
             end
           end
         end
+
         @threads << t
         stat(
           :spawned,