gnu-config: make scripts executable

This is quite convenient when using config.guess and config.sub
interactively to get a sense of autoconf's mysteries.

pkgs/development/libraries/gnu-config/default.nix

···
     mkdir -p $out
     cp ${configGuess} $out/config.guess
     cp ${configSub} $out/config.sub
+
+    chmod +x $out/config.*
   '';
 
   meta = with lib; {