build,test: fix build-addons dependency chain

* Make the 'extract embedded addons in the documentations' step a normal
  prerequisite.  As an order-only prerequisite, it's sometimes skipped
  when it shouldn't be.

* Make `tools/doc/addon-verify.js` a dependency of that step.  Changes
  to that file should result in a rebuild.

PR-URL: #6652
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Makefile

@@ -133,9 +133,9 @@ test/gc/node_modules/weak/build/Release/weakref.node: $(NODE_EXE)
 		--nodedir="$(shell pwd)"
 
 # Implicitly depends on $(NODE_EXE), see the build-addons rule for rationale.
-test/addons/.docbuildstamp: doc/api/addons.md
+test/addons/.docbuildstamp: tools/doc/addon-verify.js doc/api/addons.md
 	$(RM) -r test/addons/??_*/
-	$(NODE) tools/doc/addon-verify.js
+	$(NODE) $<
 	touch $@
 
 ADDONS_BINDING_GYPS := \
@@ -146,7 +146,7 @@ ADDONS_BINDING_GYPS := \
 test/addons/.buildstamp: $(ADDONS_BINDING_GYPS) \
 	deps/uv/include/*.h deps/v8/include/*.h \
 	src/node.h src/node_buffer.h src/node_object_wrap.h \
-	| test/addons/.docbuildstamp
+	test/addons/.docbuildstamp
 	# Cannot use $(wildcard test/addons/*/) here, it's evaluated before
 	# embedded addons have been generated from the documentation.
 	for dirname in test/addons/*/; do \