build: allow unconditionally enabling Modem.Command() method

If ModemManager is being used in a custom system, make it easy to
unconditionally enable the Modem.Command() method, in case the modem
needs to be configured in some way that is not supported via standard
modem interfaces.

src/mm-iface-modem.c

@@ -753,6 +753,7 @@ handle_command_auth_ready (MMBaseModem *self,
         return;
     }
 
+#if ! defined WITH_AT_COMMAND_VIA_DBUS
     /* If we are not in Debug mode, report an error */
     if (!mm_context_get_debug ()) {
         g_dbus_method_invocation_return_error (ctx->invocation,
@@ -763,6 +764,7 @@ handle_command_auth_ready (MMBaseModem *self,
         handle_command_context_free (ctx);
         return;
     }
+#endif
 
     /* If command is not implemented, report an error */
     if (!MM_IFACE_MODEM_GET_INTERFACE (self)->command ||