message: Update ffmpeg to use stub .so files
  So that build could always succeed. In runtime system .so files will be used.
diff: @@ -43,6 +43,7 @@ cc_library(
  defines = [],
  includes = [],
  linkopts = [
  + "-L$(GENDIR)/external/ffmpeg",
  "-l:libavformat.so.57",
  "-l:libavcodec.so.57",
  "-l:libavutil.so.55",
  @@ -50,6 +51,39 @@ cc_library(
  ],
  visibility = ["//visibility:public"],
  deps = [],
  + data = [
  + "libavformat.so.57",
  + "libavcodec.so.57",
  + "libavutil.so.55",
  + "libswscale.so.4",
  + ],
  +)
  +
  +# Stab library files for build to be successful
  +# even when those files are not installed (e.g., Ubuntu 14.04)
  +# In runtime (e.g., Ubuntu 18.04) system files will be used.
  +genrule(
  + name = "libavformat_so_57",
  + outs = ["libavformat.so.57"],
  + cmd = "echo '' | g++ -shared -fPIC -x c++ - -o $@",
  +)
  +
  +genrule(
  + name = "libavcodec_so_57",
  + outs = ["libavcodec.so.57"],
  + cmd = "echo '' | g++ -shared -fPIC -x c++ - -o $@",
  +)
  +
  +genrule(
  + name = "libavutil_so_55",
  + outs = ["libavutil.so.55"],
  + cmd = "echo '' | g++ -shared -fPIC -x c++ - -o $@",
  +)
  +
  +genrule(
  + name = "libswscale_so_4",
  + outs = ["libswscale.so.4"],
  + cmd = "echo '' | g++ -shared -fPIC -x c++ - -o $@",
  )
  genrule(
  