build: add MAKEFLAGS="-j1" to node-gyp

Currently, when building the addons the following warning is displayed:
make[2]: warning: jobserver unavailable: using -j1.  Add `+' to parent
make rule.

Adding the MAKEFLAGS="-j1" to avoid the warning.

Also updated the log message to say that it is building the addon and
not running the test as I think that is more accurate.

PR-URL: #9450
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Makefile

@@ -163,9 +163,10 @@ test/addons/.buildstamp: config.gypi \
 	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 \
-		echo "\nRunning addons test $$PWD/$$dirname" ; \
-		$(NODE) deps/npm/node_modules/node-gyp/bin/node-gyp --loglevel=$(LOGLEVEL) rebuild \
+	@for dirname in test/addons/*/; do \
+		echo "\nBuilding addon $$PWD/$$dirname" ; \
+		env MAKEFLAGS="-j1" $(NODE) deps/npm/node_modules/node-gyp/bin/node-gyp \
+		        --loglevel=$(LOGLEVEL) rebuild \
 			--python="$(PYTHON)" \
 			--directory="$$PWD/$$dirname" \
 			--nodedir="$$PWD" || exit 1 ; \