cmake config is missing version info

[bjornstromberg]

see cmake-packages for version file and the package config helpers on how to quickly add this file, since there been a few major bumps in ~6 months in this project a version file might be a good thing.

[redboltz]

Thank you for the comment. I just glanced the document.

Unfortunately, I don't have much time to improve cmake. I guuess that I am not familier to cmake very much so I couldn't understand the concept of version file. Cmake usually very difficult to understand for me ;)

If you would create a pull request to add the version file (or cmake automatically create it if the top of CMakeLists.txt has some command??), it would helpful.

async_mqtt supports vcpkg. It seems that cmake related file would affect to vcpkg (I am not sure).
Please check https://github.com/microsoft/vcpkg/tree/master/ports when you would create the pull request.