Include paths for dependent projects

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 78eea06db61c29eeec72363e3a1d7345d5fc114d..5d7c9f431a94e233e6e5d40ab607a7761567d7bd 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2,8 +2,16 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# This config is applied to targets that depend on libvpx.
+config("libvpx_external_config") {
+  include_dirs = [
+    "//third_party/libvpx_new/source/libvpx",
+  ]
+}
+
 static_library("libvpx") {
   deps = [
     "//third_party/libvpx_new",
   ]
+  public_configs = [ ":libvpx_external_config" ]
 }