make-derivation: don’t add host suffix if there is no c compiler

Some trivial builders use the name attr to choose the exec name
produced. For example nixos-install,

  nixos-install = makeProg {
    name = "nixos-install";
    src = ./nixos-install.sh;
    nix = config.nix.package.out;
    path = makeBinPath [ nixos-enter ];
  };

When cross compiling, this puts the prog in,

  /bin/nixos-install-powerpc64le-unknown-linux-gnu