build: make python executable configurable

test/cctest/cctest.gyp

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 170 matching lines @@
         {
           'action_name': 'js2c',
           'inputs': [
             '../../tools/js2c.py',
             '<@(file_list)',
           ],
           'outputs': [
             '<(generated_file)',
           ],
           'action': [
-            'python',
+            '<(python)',
             '../../tools/js2c.py',
             '<@(_outputs)',
             'TEST',  # type
             'off',  # compression
             '<@(file_list)',
           ],
         }
       ],
     },
   ],
 }