Derive default for ar_host from ar.

pylib/gyp/generator/ninja.py

 # Copyright (c) 2013 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 collections
 import copy
 import hashlib
 import json
 import multiprocessing
 import os.path
@@ skipping 1818 matching lines @@
     ld_host = '$ld'
   else:
     ar = 'ar'
     cc = 'cc'
     cxx = 'c++'
     ld = '$cc'
     ldxx = '$cxx'
     ld_host = '$cc_host'
     ldxx_host = '$cxx_host'
 
-  ar_host = 'ar'
+  ar_host = ar
   cc_host = None
   cxx_host = None
   cc_host_global_setting = None
   cxx_host_global_setting = None
   clang_cl = None
   nm = 'nm'
   nm_host = 'nm'
   readelf = 'readelf'
   readelf_host = 'readelf'
 
@@ skipping 551 matching lines @@
           arglists.append(
               (target_list, target_dicts, data, params, config_name))
         pool.map(CallGenerateOutputForConfig, arglists)
       except KeyboardInterrupt, e:
         pool.terminate()
         raise e
     else:
       for config_name in config_names:
         GenerateOutputForConfig(target_list, target_dicts, data, params,
                                 config_name)