https://nixos.wiki/wiki/OpenVPN

https://github.com/pltanton/nix-overlays/blob/master/overrides/openvpn.nix
    self: super: 
    {
      openvpn = super.lib.overrideDerivation super.openvpn (drv: {
        configureFlags = drv.configureFlags ++ ["--enable-pkcs11"];
        patches = [ ./openvpn-bug538-workaround.patch ];
        buildInputs = drv.buildInputs ++ [ self.pkcs11helper ];
      });
    }