Add support for "else if" in gyp conditions

test/conditions/elseif/elseif_bad2.gyp

+# Copyright (c) 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Trigger an error because of two consecutive string conditions, even if the
+# conditions are not actually evaluated.
+
+{
+  'targets': [
+    {
+      'variables': { 'test_var': 0 },
+      'target_name': 'program',
+      'type': 'executable',
+      'sources': [ 'program.cc' ],
+      'conditions': [
+        ['test_var==0', {
+        }, 'test_var==1', 'test_var==2', {
+        }],
+      ],
+    },
+  ],
+}