Replace comment with compile-time error and warning

wled00/mqtt.cpp

@@ -7,6 +7,10 @@
 #ifndef WLED_DISABLE_MQTT
 #define MQTT_KEEP_ALIVE_TIME 60    // contact the MQTT broker every 60 seconds
 
+#if MQTT_MAX_TOPIC_LEN > 32
+#warning "MQTT topics length > 32 is not recommended for compatibility with usermods!"
+#endif
+
 static void parseMQTTBriPayload(char* payload)
 {
   if      (strstr(payload, "ON") || strstr(payload, "on") || strstr(payload, "true")) {bri = briLast; stateUpdated(CALL_MODE_DIRECT_CHANGE);}

wled00/wled.h

@@ -476,7 +476,7 @@ WLED_GLOBAL uint16_t pollReplyCount _INIT(0);                     // count numbe
 WLED_GLOBAL unsigned long lastMqttReconnectAttempt _INIT(0);  // used for other periodic tasks too
 #ifndef WLED_DISABLE_MQTT
   #ifndef MQTT_MAX_TOPIC_LEN
-    #define MQTT_MAX_TOPIC_LEN 32  // should not be less than 32. might cause trouble when increased with usermods active that do not handle this correctly.
+    #define MQTT_MAX_TOPIC_LEN 32
   #endif
   #ifndef MQTT_MAX_SERVER_LEN
     #define MQTT_MAX_SERVER_LEN 32

wled00/wled_eeprom.cpp

@@ -2,6 +2,10 @@
 #include <EEPROM.h>
 #include "wled.h"
 
+#if defined(WLED_ENABLE_MQTT) && MQTT_MAX_TOPIC_LEN < 32
+#error "MQTT topics length < 32 is not supported by the EEPROM module!"
+#endif
+
 /*
  * DEPRECATED, do not use for new settings
  * Only used to restore config from pre-0.11 installations using the deEEP() methods