Add an option to prune unwanted targets

test/prune_targets/test2.gyp

Index: test/prune_targets/test2.gyp
diff --git a/test/prune_targets/test2.gyp b/test/prune_targets/test2.gyp
new file mode 100644
index 0000000000000000000000000000000000000000..91a437eac608f7a350f64cd0f1259d1ced470b00
--- /dev/null
+++ b/test/prune_targets/test2.gyp
@@ -0,0 +1,24 @@
+# Copyright (c) 2013 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': 'lib_wanted',
+      'type': 'static_library',
+      'sources': [ 'lib.cc' ],
+      'dependencies': [ 'lib_wanted2' ],
+    },
+    {
+      'target_name': 'lib_wanted2',
+      'type': 'static_library',
+      'sources': [ 'lib2.cc' ],
+    },
+    {
+      'target_name': 'lib_unwanted',
+      'type': 'static_library',
+      'sources': [ 'lib.cc' ],
+    },
+  ],
+}