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 ; \