build: make 'npm install' work reliably and issue build warning when node_modules look stale

This is done a in bit kludgy way on purpose so that it works on Windows and CI.

Works around npm/npm#8452.

Closes #2038

tools/npm/check-node-modules

@@ -0,0 +1,7 @@
+#!/usr/bin/env node
+
+var checkNpm = require('./check-node-modules.js');
+
+var purgeIfStale = (process.argv.indexOf('--purge') !== -1)
+
+checkNpm(true, purgeIfStale);