gyp: chromium.gyp_env behaves like .gyp/include.gypi

build/gyp_helper.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This file helps gyp_chromium and landmines correctly set up the gyp
 # environment from chromium.gyp_env on disk
 
 import os
 
 SCRIPT_DIR = os.path.dirname(os.path.realpath(__file__))
@@ skipping 24 matching lines @@
       'GYP_GENERATOR_OUTPUT',
       'GYP_GENERATORS',
       'GYP_INCLUDE_FIRST',
       'GYP_INCLUDE_LAST',
       'GYP_MSVS_VERSION',
   )
   for var in supported_vars:
     file_val = file_data.get(var)
     if file_val:
       if var in os.environ:
-        print 'INFO: Environment value for "%s" overrides value in %s.' % (
-            var, os.path.abspath(file_path)
+        behavior = 'replaces'
+        if var == 'GYP_DEFINES':
+          os.environ[var] = file_val + ' ' + os.environ[var]
+          behavior = 'overrides'
+        print 'INFO: Environment value for "%s" %s value in %s' % (
+            var, behavior, os.path.abspath(file_path)
         )
       else:
         os.environ[var] = file_val
 
 
 def apply_chromium_gyp_env():
   if 'SKIP_CHROMIUM_GYP_ENV' not in os.environ:
     # Update the environment based on chromium.gyp_env
     path = os.path.join(os.path.dirname(CHROME_SRC), 'chromium.gyp_env')
     apply_gyp_environment_from_file(path)