Makefile: include .depend as a dependency

Use -include directive to tell Make that Make itself has to create the
.depend file. Therefore, add a .depend target in addition to the phony
depend target.

Makefile

@@ -17,7 +17,7 @@
 NATIVE := true
 
 # If defined, the dependencies will be updated automatically.
-CHECKDEP :=
+CHECKDEP := 1
 
 # If defined, it indicates the number of cores
 # that make should use (e.g. -j2 for two cores).
@@ -107,12 +107,11 @@ clean:
 
 
 .PHONY: depend
-depend:
-	$(OCAMLDEP) $(SOURCESMLI) $(SOURCESML) > .depend
+depend: .depend
 
 ifdef CHECKDEP
 .depend: $(SOURCESMLI) $(SOURCESML)
-	$(OCAMLDEP) $(SOURCESMLI) $(SOURCESML) > .depend
+	$(OCAMLDEP) $(SOURCESMLI) $(SOURCESML) > $@
 endif
 
-include .depend
+-include .depend