Set OS=android via GYP_DEFINES

build/android.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 48 matching lines @@
         'android_stlport_include': '<(android_stlport)/stlport',
         'android_stlport_libs': '<(android_stlport)/libs',
       }],
     ],
     # Enable to use the system stlport, otherwise statically
     # link the NDK one?
     'use_system_stlport%': '<(android_webview_build)',
     'android_stlport_library': 'stlport_static',
     # Copy it out one scope.
     'android_webview_build%': '<(android_webview_build)',
-    'OS': 'android',
   },  # variables
   'target_defaults': {
     'defines': [
       'ANDROID',
       'V8_ANDROID_LOG_STDOUT',
     ],
     'configurations': {
       'Release': {
         'cflags': [
           '-fomit-frame-pointer',
@@ skipping 198 matching lines @@
         'ldflags!': [
           '-Wl,-z,noexecstack',
           '-Wl,--gc-sections',
           '-Wl,-O1',
           '-Wl,--as-needed',
         ],
       }],
     ],  # target_conditions
   },  # target_defaults
 }