Get ninja working for nacl.

test/configurations/x64/configurations.gyp

 # Copyright (c) 2009 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.
 
 {
   'target_defaults': {
     'configurations': {
       'Debug': {
         'msvs_configuration_platform': 'Win32',
       },
       'Debug_x64': {
         'inherit_from': ['Debug'],
         'msvs_configuration_platform': 'x64',
       },
     },
   },
   'targets': [
     {
       'target_name': 'configurations',
       'type': 'executable',
       'sources': [
         'configurations.c',
       ],
     },
+    {
+      'target_name': 'configurations64',
+      'type': 'executable',
+      'sources': [
+        'configurations.c',
+      ],
+      'configurations': {
+        'Debug': {
+          'msvs_target_platform': 'x64',
+        },
+      },
+    },
   ],
 }