node@6 6.16.0

[chrmoritz]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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continuation of #34674

This bumps the MACOSX_DEPLOYMENT_TARGET in common.gypi to the actual macOS version to avoid building against the deprecated (and in XCode 10.1 removed) libstdc++ instead of libc++, which would break building native addons.
We also need to set the nodedir npm config options so that our common.gypi (and headers from include) are used by node-gyp instead of the ones downloaded on demand from the official header tarball (which still has the old MACOSX_DEPLOYMENT_TARGET set in their common.gypi).

[fxcoudert]

Looking good, thanks @chrmoritz