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).
This capability is useful when building variations of Node.js for the same platform/architecture. For example, when building x64 Linux with FIPS compliant crypto provider, it is useful to have a "-fips" suffix to easily identify this functionality without having to examine the contents.
Contrast: node-v6.0.0-linux-x64.tar.gz vs node-v6.0.0-linux-x64-variation.tar.gz