Generate shim headers when using system v8.

tools/gyp/v8.gyp

Index: tools/gyp/v8.gyp
===================================================================
--- tools/gyp/v8.gyp	(revision 13364)
+++ tools/gyp/v8.gyp	(working copy)
@@ -991,6 +991,47 @@
               'toolsets': ['target'],
             }],
           ],
+          'variables': {
+            'shim_headers_path': '<(SHARED_INTERMEDIATE_DIR)/shim_headers/<(_target_name)/<(_toolset)',
+          },
+          'include_dirs++': [
+            '<(shim_headers_path)',
+          ],
+          'direct_dependent_settings': {
+            'include_dirs+++': [
+              '<(shim_headers_path)',
+            ],
+          },
+          'actions': [
+            {
+              'variables': {
+                'generator_path': '../../../tools/generate_shim_headers/generate_shim_headers.py',
+                'generator_args': [
+                  '--headers-root', '../../include',
+                  '--output-directory', '<(shim_headers_path)',
+                  'v8-debug.h',
+                  'v8-preparser.h',
+                  'v8-profiler.h',
+                  'v8-testing.h',
+                  'v8.h',
+                  'v8stdint.h',
+                ],
+              },
+              'action_name': 'generate_<(_target_name)_shim_headers',
+              'inputs': [
+                '<(generator_path)',
+              ],
+              'outputs': [
+                '<!@pymod_do_main(generate_shim_headers <@(generator_args) --outputs)',
+              ],
+              'action': ['python',
+                         '<(generator_path)',
+                         '<@(generator_args)',
+                         '--generate',
+              ],
+              'message': 'Generating <(_target_name) shim headers.',
+            },
+          ],
           'link_settings': {
             'libraries': [
               '-lv8',