move version number to build-utils

Summary:
* The generation of the version number is moved to build-utils so it can be used from the headless build process as well.
* The prelude script for the headless version exposes the version number on `global.__VERSION__`
* While at this file, moving some functions in the build process to use the Sync version. Doesn't matter for the build process anyways and the syntax is much nicer.

Reviewed By: passy

Differential Revision: D13843678

fbshipit-source-id: 7df8364044ab6ebc83a4060aab2e3a42ae48a934

scripts/build-utils.js

@@ -86,9 +86,19 @@ function buildFolder() {
   }).catch(die);
 }
 
+function getVersionNumber() {
+  let {version} = require('../package.json');
+  const buildNumber = process.argv.join(' ').match(/--version=(\d+)/);
+  if (buildNumber && buildNumber.length > 0) {
+    version = [...version.split('.').slice(0, 2), buildNumber[1]].join('.');
+  }
+  return version;
+}
+
 module.exports = {
   buildFolder,
   compile,
   die,
   compileDefaultPlugins,
+  getVersionNumber,
 };