Compile two H.264 files only when external FFMPEG is specified. (#197)

Author: jianjunz

modules/video_coding/BUILD.gn

@@ -500,15 +500,20 @@ rtc_library("webrtc_h264") {
   visibility = [ "*" ]
   sources = [
     "codecs/h264/h264.cc",
-    "codecs/h264/h264_color_space.cc",
-    "codecs/h264/h264_color_space.h",
-    "codecs/h264/h264_decoder_impl.cc",
-    "codecs/h264/h264_decoder_impl.h",
     "codecs/h264/h264_encoder_impl.cc",
     "codecs/h264/h264_encoder_impl.h",
     "codecs/h264/include/h264.h",
   ]
 
+  if (owt_ffmpeg_root != "") {
+    sources += [
+      "codecs/h264/h264_color_space.cc",
+      "codecs/h264/h264_color_space.h",
+      "codecs/h264/h264_decoder_impl.cc",
+      "codecs/h264/h264_decoder_impl.h",
+    ]
+  }
+
   defines = []
   include_dirs = []