Reading your cmake file I have some doubts with some path containing spaces. Is it really something for Linux ?

FYI :https://github.com/ebachard/ffmpeg-cpp

As you can see, the linux version exists since a long while. I just protected my work from "pillage" (sorry french word ... could it be looting ?). + I added some proofs of concepts who could be ported on Windows without too much changes (some are obvious).

Reading your cmake file I have some doubts with some path containing spaces. Is it really something for Linux ?
yes it is.
It works at least for linux mint 20

I made it quickly because I needed a ffmpeg wrapper for a small tool
I am not a cmake specialist so there are for sure things that could be done better (for ex if ffmpeg is installed with package or from the source)
which line is disturbing you ?

PS: pillage is looting as far as I know (french is my mother language so my english is not a reference)