build: make python executable configurable

src/d8.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 81 matching lines @@
         {
           'action_name': 'd8_js2c',
           'inputs': [
             '../tools/js2c.py',
             '<@(js_files)',
           ],
           'outputs': [
             '<(SHARED_INTERMEDIATE_DIR)/d8-js.cc',
           ],
           'action': [
-            'python',
+            '<(python)',
             '../tools/js2c.py',
             '<@(_outputs)',
             'D8',
             'off',  # compress startup data
             '<@(js_files)'
           ],
         },
       ],
     }
   ],
 }