Avoid Environment#process race condition

Race condition could have occurred if the last threaded command
was gathered in the non-blocking wait and then a blocking wait was
started because there was no job to run. Do not do a blocking wait
in this case.

lib/rscons/environment.rb

@@ -317,7 +317,7 @@ module Rscons
           end
 
           # If needed, do a blocking wait.
-          if job.nil? or @threaded_commands.size >= Rscons.n_threads
+          if (completed_tcs.empty? and job.nil?) or @threaded_commands.size >= Rscons.n_threads
             completed_tcs << wait_for_threaded_commands
           end