gyp: fix build with python 2.6

Fixes: nodejs/node-v0.x-archive#6859
PR-URL: #1325
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>

Author: indutny

tools/gyp/pylib/gyp/mac_tool.py

@@ -603,7 +603,8 @@ def _ExpandVariables(self, data, substitutions):
     if isinstance(data, list):
       return [self._ExpandVariables(v, substitutions) for v in data]
     if isinstance(data, dict):
-      return {k: self._ExpandVariables(data[k], substitutions) for k in data}
+      return dict((k, self._ExpandVariables(data[k],
+                                            substitutions)) for k in data)
     return data
 
 if __name__ == '__main__':