add login-proxy option

There are cases where systemd-logind is not used/running. Result is
that bcklight module will not run.

Add an option that, when set to false, allows backlight module to
work without systemd-logind.

src/util/backlight_backend.cpp

@@ -150,6 +150,7 @@ BacklightBackend::BacklightBackend(std::chrono::milliseconds interval,
     throw std::runtime_error("No backlight found");
   }
 
+#ifdef HAVE_LOGIN_PROXY
   // Connect to the login interface
   login_proxy_ = Gio::DBus::Proxy::create_for_bus_sync(
       Gio::DBus::BusType::BUS_TYPE_SYSTEM, "org.freedesktop.login1",
@@ -160,6 +161,7 @@ BacklightBackend::BacklightBackend(std::chrono::milliseconds interval,
         Gio::DBus::BusType::BUS_TYPE_SYSTEM, "org.freedesktop.login1",
         "/org/freedesktop/login1/session/self", "org.freedesktop.login1.Session");
   }
+#endif
 
   udev_thread_ = [this] {
     std::unique_ptr<udev, UdevDeleter> udev{udev_new()};