ninja/mac: Don't link c-file-only targets to libstdc++

test/make_global_settings/env-wrapper/wrapper.gyp

 # 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.
 
 {
   'make_global_settings': [
     ['CC', 'clang'],
-    ['LINK', 'clang++'],
     ['CC.host', 'clang'],
   ],
   'targets': [
     {
       'target_name': 'test',
       'type': 'static_library',
       'sources': [ 'foo.c' ],
     },
   ],
 }