Use a group() instead of a static_library()

BUILD.gn

 # Copyright 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.
 
-static_library("libvpx") {
-  deps = [
-    "//third_party/libvpx_new",
-  ]
+# This config is applied to targets that depend on libvpx.
+config("libvpx_external_config") {
   include_dirs = [
     "//third_party/libvpx_new/source/libvpx",
   ]
 }
+
+group("libvpx") {
+  public_deps = [
+    "//third_party/libvpx_new",
+  ]
+  public_configs = [ ":libvpx_external_config" ]
+}