Treewide: use HTTPS on GNU domains

HTTP -> HTTPS for :
- http://gnu.org/
- http://www.gnu.org/
- http://elpa.gnu.org/
- http://lists.gnu.org/
- http://gcc.gnu.org/
- http://ftp.gnu.org/ (except in fetchurl mirrors)
- http://bugs.gnu.org/

pkgs/tools/graphics/plotutils/default.nix

@@ -49,7 +49,7 @@ stdenv.mkDerivation rec {
          graphics.
       '';
 
-    homepage = http://www.gnu.org/software/plotutils/;
+    homepage = https://www.gnu.org/software/plotutils/;
 
     license = stdenv.lib.licenses.gpl2Plus;
     maintainers = [ stdenv.lib.maintainers.marcweber ];