Description
Does not compile on the ESP8266 due to ESP8266-specific code in the ArduinoMqttClient library.
There is a pull request fixing this issue arduino-libraries/ArduinoMqttClient#33 . This seems to be related to the ESP8266 core 2.5.0 release, which is long outdated.
To fix this issue locally remove the ESP8266-specific code as done in the pull request.