ninja windows: support precompiled headers

test/win/precompiled/hello.gyp

 # Copyright (c) 2012 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.
 
 {
   'targets': [
     {
       'target_name': 'hello',
       'type': 'executable',
       'sources': [
         'hello.c',
         'hello2.c',
         'precomp.c',
       ],
       'msvs_precompiled_header': 'stdio.h',
       'msvs_precompiled_source': 'precomp.c',
 
       # Required so that the printf actually causes a build failure
       # if the pch isn't included.
       'msvs_settings': {
         'VCCLCompilerTool': {
           'WarningLevel': '3',
           'WarnAsError': 'true',
         },
       },
     },
   ],
 }