ACTS-27 cmake options followup

This is the followup MR discussed in !47 (merged).

• OpenMP is on by default, unless you are using OSX.
• Added add_subdirectory_if command to simplify handling of optional components. This also prints automatic messages about enabled/disabled components.
• Consistently use build flags.

As already discussed in !47 (merged), the build flags are used to selectively include subdirectories instead of manually wrapping large parts of the CMakeLists.txt in a if()/endif() statement. There are a few cases where this was not quite possible, e.g. Examples/Fatras. In the long run, this might be split into Examples/Fatras which requires Pythia8 and Examples/FatrasDD4hep which requires Pythia8 and DD4hep.

mentioned in merge request !47 (merged)

added 1 commit

• ff9d5d44 - cmake: minor fixes

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resolved all discussions

This version looks good to me. @asalzbur, are you fine with this lighter-weight add_subdirectory_if syntax, or would you like to pursue our previous discussion on !47 (merged) regarding this matter?

I am perfectly happy with that, I think the add_subdirectory_if makes it clear right away.

Ok from my side - @hgraslan I think you can accept and merge.

Actually, I cannot ("Ask someone with write access to this repository to merge this request."). Maybe now would be a good time to ponder why?

Ok, I figured it out by re-reading the error message. It's just that I'm not one of those who can push to master in this repo, and I would need that permission to merge other peoples' branches. Makes sense.

I think only Andi and Christian could accept the the MR.

I tried to change the permissions, but failed - will check with @cgumpert next week

@asalzbur But you should be able to accept the merge request?

I can accept if approved and needed.

The test framework has no approval system enabled currently, but I would say that three people seem to be happy with this merge request judging from the comments.

merged

mentioned in commit a21ecf5d