used event loop only if defined in supported features (enabled by macros) for backward compatibily with older IDFs

include/mqtt_client.h

@@ -19,6 +19,12 @@
 extern "C" {
 #endif
 
+#ifndef ESP_EVENT_DECLARE_BASE
+// Define event loop types if macros not available
+typedef void * esp_event_loop_handle_t;
+typedef void * esp_event_handler_t;
+#endif
+
 typedef struct esp_mqtt_client *esp_mqtt_client_handle_t;
 
 /**

include/mqtt_supported_features.h

@@ -27,10 +27,15 @@
  */
 
 #ifdef ESP_IDF_VERSION
+
+#if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(3, 3, 0)
+// Features supported from 3.3
+#define MQTT_SUPPORTED_FEATURE_EVENT_LOOP
+#endif
+
 #if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(4, 0, 0)
-
+// Features supported in 4.0
 #define MQTT_SUPPORTED_FEATURE_WS_SUBPROTOCOL
-
 #endif
 #endif