Properly clean up .o files in make clean

Author: marcoroth

Makefile

@@ -90,7 +90,7 @@ test: $(test_objects) $(non_main_objects)
 
 clean:
 	rm -f $(exec) $(test_exec) $(lib_name) $(ruby_extension)
-	rm -rf src/*.o test/*.o lib/erbx/*.bundle tmp
+	rm -rf $(objects) $(test_objects) lib/erbx/*.bundle tmp
 
 bundle_install:
 	bundle install