build: adding config.gypi dep to addons/.buildstamp

Currently the build-addons target is called as part of the
test target, and it has test/addons/.buildstamp as a dependency.
When running ./configure --debug and later switching/
updating branches you can be in a situation where the config.gypi
files in the addons build directories are using an incorrect
value for default_configuration. Currently this can happen and you
will get test errors as there are a few test that depend on the
value of default_configuration to be Release.

Ben Noordhuis provided the solution for this by adding a dependency to
config.gypi, which is generated when running ./configure, and will
correct this situation.

This commit is related to #7860. Please see the dicussion in that
issue regarding the test using hard-coded paths.

Ref: https://github.com/nodejs/node/pull/8905
PR-URL: https://github.com/nodejs/node/pull/7893
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Makefile

@@ -147,7 +147,8 @@ ADDONS_BINDING_GYPS := \
 # Implicitly depends on $(NODE_EXE), see the build-addons rule for rationale.
 # Depends on node-gyp package.json so that build-addons is (re)executed when
 # node-gyp is updated as part of an npm update.
-test/addons/.buildstamp: deps/npm/node_modules/node-gyp/package.json \
+test/addons/.buildstamp: config.gypi \
+	deps/npm/node_modules/node-gyp/package.json \
 	$(ADDONS_BINDING_GYPS) test/addons/.docbuildstamp
 	# Cannot use $(wildcard test/addons/*/) here, it's evaluated before
 	# embedded addons have been generated from the documentation.