sxiv: update to 1.3-git and fix runtime behaviour

The fix involves using imlib2 built with giflib instead of a libungif.
Initially I split imlib2 into two but other distros seem to build imlib2
with giflib already and everything still builds with giflib version so I
migrated imlib2 to giflib all together.

Closes #4622

pkgs/development/libraries/imlib2/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, x11, libjpeg, libtiff, libungif, libpng, bzip2, pkgconfig }:
+{ stdenv, fetchurl, x11, libjpeg, libtiff, giflib, libpng, bzip2, pkgconfig }:
 
 stdenv.mkDerivation rec {
   name = "imlib2-1.4.6";
@@ -8,10 +8,14 @@ stdenv.mkDerivation rec {
     sha256 = "0x1j0ylpclkp8cfpwfpkjywqz124bqskyxbw8pvwzkv2gmrbwldg";
   };
 
-  buildInputs = [ x11 libjpeg libtiff libungif libpng bzip2 ];
+  buildInputs = [ x11 libjpeg libtiff giflib libpng bzip2 ];
 
   nativeBuildInputs = [ pkgconfig ];
 
+  # From
+  # https://github.com/PhantomX/slackbuilds/blob/master/imlib2/patches/imlib2-giflib51.patch
+  patches = [ ./giflib51.patch ];
+
   preConfigure = ''
     substituteInPlace imlib2-config.in \
       --replace "@my_libs@" ""