build: ensure paths with spaces can be used

Ensure include path is relative to process working directory (PWD)

This allows the use of parent paths that contain whitespace, plus
keeps the approach consistent with that used by nan.

(The previous approach of adding double quotes did not work as
intended due to node-gyp removing these on the way through.)

PR-URL: #757
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>

index.js

@@ -1,8 +1,10 @@
 const path = require('path');
 
+const include = path.relative('.', __dirname);
+
 module.exports = {
-  include: `"${__dirname}"`,
-  gyp: path.join(__dirname, 'node_api.gyp:nothing'),
+  include: include,
+  gyp: path.join(include, 'node_api.gyp:nothing'),
   isNodeApiBuiltin: true,
   needsFlag: false
 };