[gnome-sdk-images/gnome-3-28] Add GstLibav using the default system ffmpeg

[Michael Catanzaro <mcatanzaro src gnome org>]

commit f6c51b998fea3580f6aa5bb11e7cede24c382ea5
Author: Thibault Saunier <tsaunier igalia com>
Date:   Sat Mar 17 14:36:58 2018 -0300

    Add GstLibav using the default system ffmpeg

 org.gnome.Sdk.json.in | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index eae726e..307d71d 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -466,6 +466,31 @@
                  }
             ]
         },
+        {
+            "name": "gstreamer-libav",
+            "build-options" : {
+                "arch" : {
+                    "i386" : {
+                        "config-opts" : [
+                            "--build=i586-unknown-linux-gnu"
+                        ]
+                    },
+                    "arm" : {
+                        "config-opts" : [
+                            "--build=arm-unknown-linux-gnueabi"
+                        ]
+                    }
+                }
+            },
+            "config-opts": ["--with-system-libav" ],
+            "sources": [
+                {
+                    "type": "archive",
+                    "url": "https://gstreamer.freedesktop.org/src/gst-libav/gst-libav-1.12.4.tar.xz";,
+                     "sha256": "2a56aa5d2d8cd912f2bce17f174713d2c417ca298f1f9c28ee66d4aa1e1d9e62"
+                }
+            ]
+        },
         {
             "name": "libcanberra",
             "sources": [