avoid adding Paho MQTT C library twice

[koalo]

If a library is included that itself uses the Paho MQTT C++ it is not possible to add Paho MQTT C++ a second time. This results in the following message

| add_library cannot create imported target "PahoMqttC::PahoMqttC" because
| another target with the same name already exists.

It is not always possible to avoid such scenario (e.g. if third party software is used), so it is better to prevent this error.

[HpLightcorner]

Have been running into the same issue - especially when building paho.mqtt.c and paho.mqtt.cpp in the same CMake top project (e.g. in a continuous integration environment, no package managers allowed etc. etc.). Also, using the current CMake build with FetchContent is more or less impossible (as CMake has its own issues with find_library and FetchContent...) I wonder if a PR would be accepted, which ads paho.mqtt.c as a GIT submodule, an additional CMake flag (e.g. PAHO_CPP_USE_INTERNAL_MQTT_C) indicates to use the submodule or find_library? Not sure if this works out of the box - but this would allow for the FetchContent dependency management workflow. What is your opinion on that, @fpagliughi ?

[strahlc]

+1
Please merge!

[fpagliughi]

Sorry for the unbelievably slow response!