Expose napi version as a node-gyp variable

For my module I want to do conditional compilation based on node-gyp feature support.

Unfortunately there’s currently no nice way to figure out at node-gyp configuration time which napi version is supported. I’m currently doing this:

  'variables': {
    'napi_version': '<!(node -e "console.log(process.versions.napi)")',
  },

But this checks napi support in the host version of nodejs, not in the nodejs build target version.

[bnoordhuis]

node-gyp isn’t the right place to embed that logic. If node exposes a napi_build_version through its common.gypi, node-gyp will automagically make it available to add-ons. I suggest moving this to nodejs/node.

[bnoordhuis]

There’s an open pull request over at nodejs/node that is this close to landing so I’ll go ahead and close out this issue.