let
  modPkgs = import <nixpkgs> {
    crossOverlays = [( self: super: {
      gcc = super.gcc.overrideAttrs (old: {
        # this stops the building
        # buildCommand = ''
        #   echo foo
        #   exit 1
        # '';

        # this doesn't stop the building
        preBuild = ''
          echo foo
          exit 1
        '';
      });
    })];
  };
in modPkgs.gcc