deref darwin bundle symlinks only when building on win32

Windows [mangles symlinks](https://github.com/electron/electron-packager/issues/71). Currently we work around this by placing several copies of the frameworks in OS X app bundles (see 1e5aa3867d). However:

- This is invalid: the framework toplevel must [only contain symlinks](https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/FrameworkAnatomy.html). `codesign` [refuses to sign](https://stackoverflow.com/questions/25969946/osx-10-9-5-code-signing-v2-signing-a-framework-with-bundle-format-is-ambiguou) this invalid structure.

- It seriously bloats the bundle.

Since there's no fix for the Windows misbehaviour, keep the workaround, but only when cross-building on win32 for darwin; and log a warning.

gulp/standalone.js

@@ -1,3 +1,4 @@
+require('colors');
 const packager = require("electron-packager");
 const path = require("path");
 const { getVersion } = require("./buildutils");
@@ -138,7 +139,11 @@ function gulptasksStandalone($, gulp) {
                         fs.chmodSync(path.join(appPath, "play.sh"), 0o775);
                     }
 
-                    if (platform === "darwin") {
+                    if (process.platform === "win32" && platform === "darwin") {
+                        console.warn("Cross-building for macOS on Windows: dereferencing symlinks.\n".red +
+                        "This will nearly double app size and make code signature invalid. Sorry!\n".red.bold +
+                        "For more information, see " + "https://github.com/electron/electron-packager/issues/71".underline);
+
                         // Clear up framework folders
                         fs.writeFileSync(
                             path.join(appPath, "play.sh"),