Introduce a notion of "v8_code" in gyp config files.

build/standalone.gypi

 # 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 47 matching lines @@
         'target_arch%': '<(host_arch)',
       },
       'host_arch%': '<(host_arch)',
       'target_arch%': '<(target_arch)',
       'v8_target_arch%': '<(target_arch)',
     },
     'host_arch%': '<(host_arch)',
     'target_arch%': '<(target_arch)',
     'v8_target_arch%': '<(v8_target_arch)',
     'werror%': '-Werror',
+
+    # .gyp files or targets should set v8_code to 1 if they build V8 specific
+    # code, as opposed to external code.  This variable is used to control such
+    # things as the set of warnings to enable, and whether warnings are treated
+    # as errors.
+    'v8_code%': 0,
+
     'conditions': [
       ['(v8_target_arch=="arm" and host_arch!="arm") or \
         (v8_target_arch=="mipsel" and host_arch!="mipsel") or \
         (v8_target_arch=="x64" and host_arch!="x64") or \
         (OS=="android")', {
         'want_separate_host_toolset': 1,
       }, {
         'want_separate_host_toolset': 0,
       }],
+      ['OS == "win"', {
+        'os_posix%': 0,
+      }, {
+        'os_posix%': 1,
+      }],
     ],
     # Default ARM variable settings.
     'armv7%': 'default',
     'arm_neon%': 0,
     'arm_fpu%': 'vfpv3',
     'arm_float_abi%': 'default',
     'arm_thumb': 'default',
   },
   'target_defaults': {
+    'variables': {
+      'v8_code%': '<(v8_code)',
+    },
     'default_configuration': 'Debug',
     'configurations': {
       'Debug': {
         'cflags': [ '-g', '-O0' ],
       },
       'Release': {
         # Xcode insists on this empty entry.
       },
     },
+    'target_conditions': [
+      ['v8_code == 0', {
+        'conditions': [
+          ['os_posix == 1 and OS != "mac"', {
+            'cflags!': [
+              '-Werror',
+            ],
+          }],
+          ['OS == "mac"', {
+            'xcode_settings': {
+              'GCC_TREAT_WARNINGS_AS_ERRORS': 'NO',    # -Werror
+            },
+          }],
+          ['OS == "win"', {
+            'msvs_settings': {
+              'VCCLCompilerTool': {
+                'WarnAsError': 'false',
+              },
+            },
+          }],
+        ],
+      }],
+    ],
   },
   'conditions': [
     ['OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris" \
        or OS=="netbsd"', {
       'target_defaults': {
         'cflags': [ '-Wall', '<(werror)', '-W', '-Wno-unused-parameter',
                     '-pthread', '-fno-exceptions', '-pedantic' ],
         'cflags_cc': [ '-Wnon-virtual-dtor', '-fno-rtti' ],
         'ldflags': [ '-pthread', ],
         'conditions': [
@@ skipping 111 matching lines @@
         },
         'target_conditions': [
           ['_type!="static_library"', {
             'xcode_settings': {'OTHER_LDFLAGS': ['-Wl,-search_paths_first']},
           }],
         ],  # target_conditions
       },  # target_defaults
     }],  # OS=="mac"
   ],
 }