Auto-negotiate MQTT version, rather than using a #define. #119
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
A few changes in Adafruit_MQTT.h and .cpp that allow connecting to any MQTT broker without knowing in advance what protocol version it uses.
The original used a hard-coded
#define MQTT_PROTOCOL_LEVELof either 3 or 4 to distinguish between the two protocol versions. In this version, mqtt_protocol_level is a member variable, and the protocol version is determined in the same (kludgy) way that Paho does it -- try to connect with it set to 4, then switch to 3 if that fails.
But this should be transparent to the calling code, aside from taking a tiny bit longer.