build: update build-addons when node-gyp changes

We can tell when `node-gyp` is changed by creating a prerequisite on
`deps/npm/node_modules/node-gyp/package.json`. The prerequisite is added
to the `test/addons/.buildstamp` since `build-addons` is .PHONY.

Testing for this change was entirely manual.

  $ make clean test-build # Initial build
  $ make test-build # Make sure build-addons doesn't rebuild
  $ touch deps/npm/node_modules/node-gyp/package.json # simulate change
  $ make test-build # Ensure build-addons rebuilds

PR-URL: https://github.com/nodejs/node/pull/6787


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>