ninja windows: support precompiled headers

test/msvs/precompiled/hello.gyp

Index: test/msvs/precompiled/hello.gyp
===================================================================
--- test/msvs/precompiled/hello.gyp	(revision 1396)
+++ test/msvs/precompiled/hello.gyp	(working copy)
@@ -1,28 +0,0 @@
-# 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',
-        },
-      },
-    },
-  ],
-}