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: https://github.com/nodejs/node/pull/9450


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