Allowing use of lambda expressions in callbacks on Wio Terminal

[matsujirushi]

I want to use of lambda expressions in callbacks on Wio Terminal.
https://github.com/knolleary/pubsubclient/blob/master/src/PubSubClient.h#L79-L84

Would you consider changing the condition of #if?

i.e.

#if __cplusplus >= 201103L

[BlackJackMC]

Hello. I wonder if you are still interested in this topic. I also have the same concern.
Why do the author need to change the callback signature on non-esp devices?

#if defined(ESP8266) || defined(ESP32)
#include <functional>
#define MQTT_CALLBACK_SIGNATURE std::function<void(char*, uint8_t*, unsigned int)> callback
#else
#define MQTT_CALLBACK_SIGNATURE void (*callback)(char*, uint8_t*, unsigned int)
#endif