CMake Build Configuration Updates, master branch (2020.03.12.)

While reviewing atlas/atlasexternals!618 (merged) I found that this code is not nearly in a state that I would really like.

I spent a bit of time to:

• Bring the CMake configuration to the same style that GeoModelCore and GeoModelIO use;
• Teach the CMake build how to build XercesC and nlohmann_json by itself, if the user asks for that;
• Created a GitLab CI configuration for building the code in a few different ways, to test these improvements.

Pinging @rbianchi and @boudreau.

Okay, apparently the CI is not in as good a shape as I thought...

added 1 commit

• 56d12b9e - Added gmp-devel to the OS, to fix the CentOS 7 standalone build.

Compare with previous version

mentioned in merge request atlas/atlasexternals!618 (merged)

added 3 commits

• 56d12b9e...6bbf0b3b - 2 commits from branch GeoModelDev:master
• d577423c - Merge remote-tracking branch 'upstream/master' into CMakeBuildUpdates-master-20200312

Compare with previous version

mentioned in merge request atlas/atlasexternals!646 (merged)

Thanks for this MR, @akraszna!! And thanks for the ping about that in atlas/atlasexternals!646 (merged) !

Yes, the status of this package was in "stub" stage concerning the CMake settings. Thanks a lot for helping in completing it.

Also, the CI was not set for this package yet. Thanks for that, too!!

Looks very good, merging now! Thanks again, @akraszna !!

merged

mentioned in commit 3d2f1494