You can remove this line and instead use installFlags = [ "PREFIX=$(out)" ]

Thanks and please excuse my late reply.

I think it would be better to not do the String -> Path conversions here, because it could be the case that the user wants to provide their own file, so they'd have to pass builtins.readFile ./my-config.def.h, which would only get converted right back to a file here.

So I suggest making conf a path parameter, removing the configFile attribute and just using cp ${conf} config.def.h. Also an assertion that checks for conf actually being a path (starts with "/") would be nice, along with a simple message.

And probably configFile would be a better name then, there's a couple uses of a configFile argument throughout nixpkgs already.

I have to admit that I'm not a big fan from this solution, which I adapted from the nixpkgs/pkgs/applications/misc/st/default.nix-file. Isn't it better if there is at least some regularity? That's the reason why I choose this way.

I see, didn't know that. While I don't like this approach because it possibly does redundant work, I think it it makes sense to have some regularity, so yeah that's okay

I like this idea, and I see config being used in other places throughout nixpkgs, but I'm not sure whether I understand the purpose, because it's just as easy to use slstatus.override { configFile = ./foobar.def.h; } instead

This allows you to set this specific config-entry in, for example, your ~/.nixpkgs/config.nix file.

{
  slstatus.conf = ''
    #define MAXLEN 2048

    static const int interval = 15000;
    static const char unknown_str[] = "NA";

    static const struct arg args[] = { … };
  '';
}