gocr: Install libraries and includes

pkgs/applications/graphics/gocr/default.nix

···
     sha256 = "1dgmcpapy7h68d53q2c5d0bpgzgfb2nw2blndnx9qhc7z12149mw";
   };
 
+  buildFlags = [ "all" "libs" ];
+  installFlags = [ "libdir=/lib/" ]; # Specify libdir so Makefile will also install library.
+
+  preInstall = "mkdir -p $out/lib";
+
   postInstall = ''
+    for i in pgm2asc.h gocr.h; do
+      install -D -m644 src/$i $out/include/gocr/$i
+    done
+  '';
+
+  preFixup = ''
     sed -i -e 's|exec wish|exec ${tk}/bin/wish|' $out/bin/gocr.tcl
   '';