Add support for "else if" in gyp conditions

test/conditions/elseif/elseif_bad1.gyp

Index: test/conditions/elseif/elseif_bad1.gyp
diff --git a/test/conditions/elseif/elseif_bad1.gyp b/test/conditions/elseif/elseif_bad1.gyp
new file mode 100644
index 0000000000000000000000000000000000000000..35c8455ccae177b7dcc75e0f14ef9db117bdd31a
--- /dev/null
+++ b/test/conditions/elseif/elseif_bad1.gyp
@@ -0,0 +1,20 @@
+# 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.
+
+{
+  'targets': [
+    {
+      'variables': { 'test_var': 0 },
+      'target_name': 'program',
+      'type': 'executable',
+      'sources': [ 'program.cc' ],
+      'conditions': [
+        ['test_var==0', 'test_var==1', {
+        }],
+      ],
+    },
+  ],
+}