shortcut Environment#process if @targets is empty

lib/rscons/environment.rb

@@ -175,6 +175,7 @@ module Rscons
       cache = Cache.instance
       cache.clear_checksum_cache!
       targets_processed = {}
+      unless @targets.empty?
         process_target = proc do |target|
           targets_processed[target] ||= begin
             @targets[target][:sources].each do |src|
@@ -199,6 +200,7 @@ module Rscons
         end
         cache.write
       end
+    end
 
     # Clear all targets registered for the Environment.
     def clear_targets