Quoting Graham Norbury <gnorbury@bondcar.com>:

> Drop back to an earlier cmake version; 3.12.4 works fine for me.
> Sorry, I'd update the cmake rules but I've never been able to figure the
> magic incantation in GrSwig.cmake to fix the incorrect dependency ordering.
>

See: CMake bug reports and follow up and issues follow up

https://discourse.cmake.org/t/newer-cmake-v-3-12-4-v-op25-project/320

https://github.com/boatbod/op25/issues/25

I've asked for what edits they made to see if it works on Raspbian, 
etc.. and will return the FAQ/HOWTO's etc. back to working status...

Did you confirm 3.12.4??? Seems others had to go back further?? 3.10.3 etc...