Fix include path when pointing to Node.js source
Fix include path when pointing to Node.js source

Header files for deps are in a different location in the Node.js
source tree compared to the release tarballs.

PR-URL: https://github.com/nodejs/node-gyp/pull/1055
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodejs/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodegit/node-gyp,nodejs/node-gyp,nodegit/node-gyp