optional ABI override in neon build

[dherman]

Currently neon build automatically sets the Node ABI version number by extracting it from process.versions.modules. It should be possible to pass an optional ABI version like neon build --abi=43 to support "cross-compiling" to different Node versions.

[shawedwa]

This makes it difficult to build Rust modules for Electron, since the ABI is typically different for Electron and one's own Node install.

[shawedwa]

It might be nice if it could pull the right version from the included node headers - NODE_MODULE_VERSION is defined in node_version.h.

PR: neon-bindings/neon#109

Edit: Will need to make a corresponding change to neon-cli to not pass in ABI version (unless explicitly overridden) .

[EdShaw]

And the corresponding change in neon-cli which actually resolves this issue:
#31

[dherman]

OK I'm closing this one, because as @EdShaw pointed out it's not safe to compile to a different ABI than the headers you're compiling with.