Calf: use gcc5 to fix the build

There hasn't been a release in a few years, but they're working on it.

pkgs/top-level/all-packages.nix

···
 
   calf = callPackage ../applications/audio/calf {
       inherit (gnome2) libglade;
+      stdenv = overrideCC stdenv gcc5;
   };
 
   calcurse = callPackage ../applications/misc/calcurse { };