thing.nix

let
  customStdenv = stdenv.override {
    cc = stdenv.cc.override {
      libc = pkgs.glibc.override rec {
        # whatever changes you want here, to get a different version
        version = "4";
        src = fetchurl {
          url = "mirror://gnu/glibc/glibc-${version}.tar.xz";
          sha256 = "aaAAAAAAAAAAaaaaaaaaAAAAAAAAaaaAAAaaaAAAaAAaaAAAAAAA";
        };
      };
    };
  };
in
customStdenv.mkDerivation {}