Ensure that helper files are built as part of make all

Makefile

@@ -35,13 +35,21 @@ clean_methods := $(methods:%=clean-%)
 .PHONY: all
 all: $(methods)
 
+.PHONY: helper
+helper:
+	$(MAKE) -C src/helper_files
+
 .PHONY: methods
-$(methods):
+$(methods): helper
 	$(MAKE) -C src/methods/$@
 
+.PHONY: clean_helper
+clean_helper:
+	$(MAKE) -C src/helper_files
+
 .PHONY: $(clean_methods)
 $(clean_methods):
 	$(MAKE) -C src/methods/$(@:clean-%=%) clean
 
 .PHONY: clean
-clean: $(clean_methods)
+clean: clean_helper $(clean_methods)