build/linux/unbundle: make ffmpeg.gn define USE_SYSTEM_FFMPEG

build/linux/unbundle/ffmpeg.gn

 # Copyright 2016 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.
 
 import("//build/config/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
 pkg_config("system_ffmpeg") {
   packages = [
     "libavcodec",
     "libavformat",
     "libavutil",
   ]
 }
 
+config("using_system_ffmpeg") {
+  defines = [ "USE_SYSTEM_FFMPEG=1" ]
+}
+
 shim_headers("ffmpeg_shim") {
   root_path = "."
   headers = [
     "libavcodec/avcodec.h",
     "libavformat/avformat.h",
     "libavutil/imgutils.h",
   ]
 }
 
 source_set("ffmpeg") {
   deps = [
     ":ffmpeg_shim",
   ]
-  public_configs = [ ":system_ffmpeg" ]
+  public_configs = [
+    ":system_ffmpeg",
+    ":using_system_ffmpeg",
+  ]
 }