deps: fix gyp to work on MacOSX without XCode

This is a port of eb459c8 ,
used as a floating patch over gyp.

Original commit message:

  This issue has already submitted to the upstream in
  https://code.google.com/p/gyp/issues/detail?id=477
  Use this commit until the upstream is to be fixed.

  PR-URL: #1325
  Reviewed-By: Fedor Indutny <fedor@indutny.com>
  Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

This was ported to v0.10 in nodejs/node-v0.x-archive#25857

PR-URL: #2843
Reviewed-By: rvagg - Rod Vagg <rod@vagg.org>
Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

Author: Shigeki Ohtsu

tools/gyp/pylib/gyp/xcode_emulation.py

@@ -1479,6 +1479,8 @@ def _GetXcodeEnv(xcode_settings, built_products_dir, srcroot, configuration,
     sdk_root = xcode_settings._SdkRoot(configuration)
     if not sdk_root:
       sdk_root = xcode_settings._XcodeSdkPath('')
+    if sdk_root is None:
+      sdk_root = ''
     env['SDKROOT'] = sdk_root
 
   if not additional_settings: