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..16f0fd3290de9c439fd7ccd085ad6cd918014ab9
--- /dev/null
+++ b/test/prune_targets/test2.gyp
@@ -0,0 +1,30 @@
+# 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': 'lib1',
+      'type': 'static_library',
+      'sources': [ 'lib1.cc' ],
+      'dependencies': [ 'lib_indirect' ],
+    },
+    {
+      'target_name': 'lib2',
+      'type': 'static_library',
+      'sources': [ 'lib2.cc' ],
+      'dependencies': [ 'lib_indirect' ],
+    },
+    {
+      'target_name': 'lib3',
+      'type': 'static_library',
+      'sources': [ 'lib3.cc' ],
+    },
+    {
+      'target_name': 'lib_indirect',
+      'type': 'static_library',
+      'sources': [ 'lib_indirect.cc' ],
+    },
+  ],
+}