build: Add VARIATION variable to binary target

If the VARIATION variable is present, then make binary will produce archives
named node-$(FULLVERSION)-$(PLATFORM)-$(ARCH)-$(VARIATION).

PR-URL: https://github.com/nodejs/node/pull/4631


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>