imagemagick: add webp support

pkgs/applications/graphics/ImageMagick/default.nix

···
 { lib, stdenv, fetchurl, pkgconfig, libtool
 , bzip2, zlib, libX11, libXext, libXt, fontconfig, freetype, ghostscript, libjpeg
-, lcms2, openexr, libpng, librsvg, libtiff, libxml2, openjpeg
+, lcms2, openexr, libpng, librsvg, libtiff, libxml2, openjpeg, libwebp
 }:
 
 let
···
 
   buildInputs =
     [ pkgconfig libtool zlib fontconfig freetype ghostscript libjpeg
-      openexr libpng librsvg libtiff libxml2 openjpeg
+      openexr libpng librsvg libtiff libxml2 openjpeg libwebp
     ];
 
   propagatedBuildInputs =
     [ bzip2 freetype libjpeg libX11 libXext libXt lcms2 ];
 
   postInstall = ''
+
     (cd "$out/include" && ln -s ImageMagick* ImageMagick)
   '' + lib.optionalString (ghostscript != null) ''
     for la in $out/lib/*.la; do

pkgs/top-level/all-packages.nix

···
     libtiff = null;
     libxml2 = null;
     openjpeg = null;
+    libwebp = null;
   };
 
   imagemagick = self.imagemagickBig.override {