cimg: fix paths and unbreak overrides

pkgs/development/libraries/cimg/builder.sh

···
-
-source $stdenv/setup
-
-unpackPhase
-cd $sourceRoot
-
-install -dm 755 $out/include/cimg $doc/share/doc/cimg/html $doc/share/cimg/examples $doc/share/cimg/plugins
-
-install -m 644 CImg.h $out/include/cimg
-cp -dr --no-preserve=ownership examples/* $doc/share/cimg/examples/
-cp -dr --no-preserve=ownership plugins/* $doc/share/cimg/plugins/

pkgs/development/libraries/cimg/default.nix

···
-{ stdenv, fetchurl
-, unzip }:
+{ stdenv, fetchurl, unzip }:
 
 stdenv.mkDerivation rec {
 
···
 
   nativeBuildInputs = [ unzip ];
 
-  builder = ./builder.sh;
+  installPhase = ''
+    install -dm 755 $out/include/CImg/plugins $doc/share/doc/cimg/examples
+
+    install -m 644 CImg.h $out/include/
+    cp -dr --no-preserve=ownership examples/* $doc/share/doc/cimg/examples/
+    cp -dr --no-preserve=ownership plugins/* $out/include/CImg/plugins/
+    cp README.txt $doc/share/doc/cimg/
+  '';
 
   outputs = [ "out" "doc" ];