Fix makefile targets.

Makefile

@@ -36,16 +36,16 @@ subdirs: $(SUBDIRS)
 $(SUBDIRS):
 	@$(MAKE) -C $@ $(MAKECMDGOALS)
 
-manifest: MANIFEST.json
-
-MANIFEST.json: FORCE
-	@$(PYTHON) tools/w3c/scripts/manifest.py
+um: update_manifest
 
-update_manifest: manifest
+update_manifest: MANIFEST.json
 	@git add MANIFEST.json
 	@git commit -m 'Update manifest.'
 
-um: update_manifest
+manifest: MANIFEST.json
+
+MANIFEST.json: FORCE
+	@$(PYTHON) tools/w3c/scripts/manifest.py
 
 FORCE: