Enable pixbuf-loader

for gtk to be able to use the pixbuf-loader for svg files it needs to be enabled, and a loaders.cache needs to be created.

pkgs/development/libraries/librsvg/default.nix

@@ -11,6 +11,19 @@ stdenv.mkDerivation {
   propagatedBuildInputs = [ glib gtk ];
   buildNativeInputs = [ pkgconfig ];
 
-  # It tries to install the loader to $gdk_pixbuf
-  configureFlags = "--disable-pixbuf-loader";
+  # It wants to add loaders and update the loaders.cache in gdk-pixbuf
+  # Patching the Makefiles to it creates rsvg specific loaders and the
+  # relevant loader.cache here.
+  # The loaders.cache can be used by setting GDK_PIXBUF_MODULE_FILE to
+  # point to this file in a wrapper.
+  postConfigure = ''
+    GDK_PIXBUF=$out/lib/gdk-pixbuf
+    mkdir -p $GDK_PIXBUF/loaders
+    sed -e "s#gdk_pixbuf_moduledir = .*#gdk_pixbuf_moduledir = $GDK_PIXBUF/loaders#" \
+        -i gdk-pixbuf-loader/Makefile
+    sed -e "s#gdk_pixbuf_cache_file = .*#gdk_pixbuf_cache_file = $GDK_PIXBUF/loaders.cache#" \
+        -i gdk-pixbuf-loader/Makefile
+    sed -e "s#\$(GDK_PIXBUF_QUERYLOADERS)#GDK_PIXBUF_MODULEDIR=$GDK_PIXBUF/loaders \$(GDK_PIXBUF_QUERYLOADERS)#" \
+         -i gdk-pixbuf-loader/Makefile
+  '';
 }