ninja windows: support precompiled headers

test/msvs/precompiled/hello.c

Index: test/msvs/precompiled/hello.c
===================================================================
--- test/msvs/precompiled/hello.c	(revision 1396)
+++ test/msvs/precompiled/hello.c	(working copy)
@@ -1,14 +0,0 @@
-/* Copyright (c) 2011 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. */
-
-// Note the abscence of a stdio.h include.  This will be inserted because of the
-// precompiled header.
-
-extern int hello2();
-
-int main(int argc, char *argv[]) {
-  printf("Hello, world!\n");
-  hello2();
-  return 0;
-}