Derive default for ar_host from ar.

test/win/lib-crosscompile/use_host_ar.gyp

-# Copyright (c) 2012 Google Inc. All rights reserved.
+# Copyright (c) 2015 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.
 
 {
  'targets': [
     {
       'target_name': 'lib_answer',
       'type': 'static_library',
+      'toolsets': ['host'],
       'msvs_settings': {
-        'VCCLCompilerTool': {
-          'WholeProgramOptimization': 'true',  # /GL
-        },
-        'VCLibrarianTool': {
-          'LinkTimeCodeGeneration': 'true',    # /LTCG
-        },
+        'msvs_cygwin_shell': 0,
       },
       'sources': ['answer.cc'],
     },
   ]
 }