I had to change the gr-osmosdr repository to compile it successfully.
I used https://github.com/igorauad/gr-osmosdr.git (it is still needed to checkout branch gr3.8).
Tested on a fresh ubuntu 18.04.4 LTS install.
Probably error was due to too old cmake version. This is now fixed, recent cmake is installed from ppa. It builds the gr3.8 branch of the official osmocom repo.
I had to change the gr-osmosdr repository to compile it successfully. I used https://github.com/igorauad/gr-osmosdr.git (it is still needed to checkout branch gr3.8). Tested on a fresh ubuntu 18.04.4 LTS install.