imlib2: fix Darwin build

pkgs/development/libraries/imlib2/default.nix

@@ -21,7 +21,23 @@ stdenv.mkDerivation rec {
       --replace "@my_libs@" ""
   '';
 
+  # Do not build amd64 assembly code on Darwin, because it fails to compile
+  # with unknow directive errors
+  configureFlags = if stdenv.isDarwin then [ "--enable-amd64=no" ] else null;
+
   meta = {
-    hydraPlatforms = stdenv.lib.platforms.linux;
+    description = "Image manipulation library";
+
+    longDescription = ''
+      This is the Imlib 2 library - a library that does image file loading and
+      saving as well as rendering, manipulation, arbitrary polygon support, etc.
+      It does ALL of these operations FAST. Imlib2 also tries to be highly
+      intelligent about doing them, so writing naive programs can be done
+      easily, without sacrificing speed.
+    '';
+
+    license = stdenv.lib.licenses.free;
+    platforms = stdenv.lib.platforms.unix;
+    maintainers = with stdenv.lib.maintainers; [ spwhitt ];
   };
 }