Deprecated exec_program calls in cmake

[truenicfel]

CMake exec_program has been deprecated for almost ten years now. This is probably covered by #133 but as there is no progress so far I recommend doing this as a first step.

Furthermore, the minimum version of cmake this project is using is also deprecated. I highly recommend updating this soon.

[truenicfel]

I have a fork which I am currently using in my project to avoid the warnings. This could be merged via pull_request anytime.

[WardF]

I'm going through our outstanding PR's now, a PR to fix this would be great, thank you!