Build sassc as a dependency for flatpaks

Downloading the subproject during the GTK build fails
in a flatpak build due to lack of network access.
flatpak-builder insists on having these things explicitly
spelled out as dependencies.

build-aux/flatpak/org.gtk.Demo4.json

@@ -1,7 +1,7 @@
 {
     "app-id": "org.gtk.Demo4",
     "runtime": "org.gnome.Platform",
-    "runtime-version": "master",
+    "runtime-version": "3.34",
     "sdk": "org.gnome.Sdk",
     "command": "gtk4-demo",
     "tags": ["devel", "development", "nightly"],
@@ -54,6 +54,36 @@
                 }
             ]
         },
+        {
+            "name" : "libsass",
+            "buildsystem" : "meson",
+            "builddir" : true,
+            "config-opts": [
+                "--libdir=/app/lib"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/lazka/libsass.git",
+                    "branch" : "meson"
+                }
+            ]
+        },
+        {
+            "name" : "sassc",
+            "buildsystem" : "meson",
+            "builddir" : true,
+            "config-opts": [
+                "--libdir=/app/lib"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/lazka/sassc.git",
+                    "branch" : "meson"
+                }
+            ]
+        },
         {
             "name": "gtk",
             "buildsystem": "meson",

build-aux/flatpak/org.gtk.IconBrowser4.json

@@ -54,6 +54,36 @@
                 }
             ]
         },
+        {
+            "name" : "libsass",
+            "buildsystem" : "meson",
+            "builddir" : true,
+            "config-opts": [
+                "--libdir=/app/lib"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/lazka/libsass.git",
+                    "branch" : "meson"
+                }
+            ]
+        },
+        {
+            "name" : "sassc",
+            "buildsystem" : "meson",
+            "builddir" : true,
+            "config-opts": [
+                "--libdir=/app/lib"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/lazka/sassc.git",
+                    "branch" : "meson"
+                }
+            ]
+        },
         {
             "name": "gtk",
             "buildsystem": "meson",

build-aux/flatpak/org.gtk.WidgetFactory4.json

@@ -54,6 +54,36 @@
                 }
             ]
         },
+        {
+            "name" : "libsass",
+            "buildsystem" : "meson",
+            "builddir" : true,
+            "config-opts": [
+                "--libdir=/app/lib"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/lazka/libsass.git",
+                    "branch" : "meson"
+                }
+            ]
+        },
+        {
+            "name" : "sassc",
+            "buildsystem" : "meson",
+            "builddir" : true,
+            "config-opts": [
+                "--libdir=/app/lib"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/lazka/sassc.git",
+                    "branch" : "meson"
+                }
+            ]
+        },
         {
             "name": "gtk",
             "buildsystem": "meson",