Need CI pipeline with cmake 3.9

We require cmake 3.9 as the minimum version of cmake. We have to detect when we use features available only in more recent versions.

Depends on having the right container first (corsika-docker#1).

mentioned in merge request !95 (closed)

Maybe the 3.9 version was too much ad-hoc? Right now we are running all CI with ubunut18.04 and cmake 3.10.2.

If I look at: https://gitlab.kitware.com/cmake/community/-/wikis/CMake-Versions-on-Linux-Distros

I am wondering if we really need to support any of those distro versions with lower versions of cmake. Lukas, what do you think? The easiest way would be to rise our cmake-minimim to ubuntu 18.04 (cmake 3.10.2), or we add another distro with cmake 3.9 to our CI.

I think the former sounds good to me.

We cannot change the requirement. It is imposed by the need to work on CentOS 7, which is the system used in many major production sites.

Actually, I was wrong. It is not the centos container, it was as low as I could go before breaking support for one of our externals. I think we can increase the version. To develop on CentOS 7, one has to install an external cmake anyways.

Ok, but which version? 3.10.2?

I mean, alternatively/or additionally it would also be fine to add a suited CentOS container on CI.

added Continuous Integration label