ci: test also cmake 3.10

[strk]

I can't build because the cmake version on ubuntu-18.04 is 3.10.2.
This PR is to check if it's possible to build there, given enough
time :)

[antanasbruzas]

According to github help ubuntu-latest is the same as ubuntu-18.04. Until they upgrade ubuntu-latest to 20.04 (which is still not known when) it means that CI will run twice on the same environment. Or am I missing something? Maybe simply lowering required cmake version without changing CI would be fine?

[antanasbruzas]

Because I had cmake --version added in the configure step I can see that even for ubuntu-18.04 it still shows cmake version 3.17.0 so there is really no gain by running CI on ubuntu-18.04 before having 20.04 available here. Please update your pull request to only inlude change for CMakeLists.txt and not .github/workflows/cmake.yml

[strk]

I reverted the change in OS matrix, but added a cmake matrix item, to test 3 cmake versions.
CI shows that 3.10 is NOT working due to:

CXX_STANDARD is set to invalid value '20'

Is there anything we can do to fix that, to improve support for older systems ? (my ubuntu 18.04 has cmake 3.10)

[strk]

For the record:

# apt-cache policy cmake
cmake:
  Installed: 3.10.2-1ubuntu2.18.04.1
  Candidate: 3.10.2-1ubuntu2.18.04.1
  Version table:
 *** 3.10.2-1ubuntu2.18.04.1 500
        500 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     3.10.2-1ubuntu2 500
        500 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

[strk]

The error with cmake 3.10 is:

CXX_STANDARD is set to invalid value '20'

Would be nice to fix, but dunno when I'll have time to look at it - please keep this PR open if you don't plan to look at it yourself sooner

[antanasbruzas]

The error with cmake 3.10 is:

CXX_STANDARD is set to invalid value '20'

Would be nice to fix, but dunno when I'll have time to look at it - please keep this PR open if you don't plan to look at it yourself sooner

develop branch has a potential fix with 66cadb5

[strk]

I've rebased this PR against the "develop" branch. I saw your change only affected linux builds though, not Windows, so I guess the CI matrix should be changed accordingly, if things fail there...

[antanasbruzas]

I've rebased this PR against the "develop" branch. I saw your change only affected linux builds though, not Windows, so I guess the CI matrix should be changed accordingly, if things fail there...

I know that windows build will fail with lower cmake versions but I don't have a windows machine to work on and it's a pain to develop for windows relying on github VM only (as you can see from commit history :))

[strk]

I've exluded windows with cmake < 3.15 from CI, linux and mac are still tested, and we're all happy :)