from source

{ lib, config, pkgs, ... }:

let
  foo = pkgs.mesa_noglu.overrideDerivation (oldAttrs: {
    src = /home/clever/x/mesa-11.1.1;
    dontStrip = true;
  });
in
{
  hardware = {
    opengl = {
      driSupport32Bit = true;
      package = pkgs.buildEnv {
        name = "custom-hack";
        paths = [
          foo foo.drivers
        ];
      };
    };
  };
}

just debug

{ lib, config, pkgs, ... }:

let
  foo = pkgs.enableDebugging pkgs.mesa_noglu;
in
{
  hardware = {
    opengl = {
      driSupport32Bit = true;
      package = pkgs.buildEnv {
        name = "custom-hack";
        paths = [
          foo foo.drivers
        ];
      };
    };
  };
}