Allow override of OS variable on Windows ninja.

pylib/gyp/generator/ninja.py

 # Copyright (c) 2012 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import copy
 import gyp
 import gyp.common
 import gyp.msvs_emulation
 import gyp.system_test
 import gyp.xcode_emulation
@@ skipping 973 matching lines @@
     global generator_additional_non_configuration_keys
     generator_additional_non_configuration_keys = getattr(xcode_generator,
         'generator_additional_non_configuration_keys', [])
     global generator_additional_path_sections
     generator_additional_path_sections = getattr(xcode_generator,
         'generator_additional_path_sections', [])
     global generator_extra_sources_for_rules
     generator_extra_sources_for_rules = getattr(xcode_generator,
         'generator_extra_sources_for_rules', [])
   elif flavor == 'win':
-    default_variables['OS'] = 'win'
+    default_variables.setdefault('OS', 'win')
     default_variables['EXECUTABLE_SUFFIX'] = '.exe'
     default_variables['STATIC_LIB_PREFIX'] = ''
     default_variables['STATIC_LIB_SUFFIX'] = '.lib'
     default_variables['SHARED_LIB_PREFIX'] = ''
     default_variables['SHARED_LIB_SUFFIX'] = '.dll'
   else:
     operating_system = flavor
     if flavor == 'android':
       operating_system = 'linux'  # Keep this legacy behavior for now.
     default_variables.setdefault('OS', operating_system)
@@ skipping 247 matching lines @@
 
   user_config = params.get('generator_flags', {}).get('config', None)
   if user_config:
     GenerateOutputForConfig(target_list, target_dicts, data, params,
                             user_config)
   else:
     config_names = target_dicts[target_list[0]]['configurations'].keys()
     for config_name in config_names:
       GenerateOutputForConfig(target_list, target_dicts, data, params,
                               config_name)