Define OS=mac in gyp when building mac tools on iOS

build/ios/mac_build.gypi

 # 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.
 
 # Xcode throws an error if an iOS target depends on a Mac OS X target. So
 # any place a utility program needs to be build and run, an action is
 # used to run ninja as script to work around this.
 # Example:
 # {
 #   'target_name': 'foo',
@@ skipping 51 matching lines @@
       # Don't use anything set for the iOS side of things.
       '--ignore-environment',
       # Generate for ninja
       '--format=ninja',
       # Generate files into xcodebuild/ninja
       '-Goutput_dir=xcodebuild/<(ninja_output_dir)',
       # nacl isn't in the iOS checkout, make sure it's turned off
       '-Ddisable_nacl=1',
       # Pass through the Mac SDK version.
       '-Dmac_sdk=<(mac_sdk)',
-      '-Dparent_generator=<(parent_generator)'
+      '-Dparent_generator=<(parent_generator)',
+      '-DOS=mac',
     ],
 
     # Rerun gyp for each of the projects needed. This is what actually
     # generates the projects on disk.
     're_run_gyp_execution':
       '<!(cd <(DEPTH) && <@(re_run_gyp) <@(re_run_targets))',
   },
   # Since these are used to generate things needed by other targets, make
   # them hard dependencies so they are always built first.
   'hard_dependency': 1,
 }