ninja windows: support alternative entrypoints.

test/win/linker-flags/entrypointsymbol.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': 'test_ok',
+      'type': 'executable',
+      'msvs_settings': {
+        'VCLinkerTool': {
+          'EntryPointSymbol': 'MainEntryPoint',
+        }
+      },
+      'sources': ['entrypointsymbol.cc'],
+    },
+    {
+      'target_name': 'test_fail',
+      'type': 'executable',
+      'msvs_settings': {
+        'VCLinkerTool': {
+          'EntryPointSymbol': 'MainEntryPoint',
+        }
+      },
+      'sources': ['hello.cc'],
+    },
+  ]
+}