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.

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>