Define minimum C++ version using ROOT_CXX_STANDARD

[denproc]

Changes in this pull request

Adjusted CMake recipe to define minimum C++ version using ROOT_CXX_STANDARD.

Testing performed

The definition of the build variables has been tested on MacOS for root==6.28,6.30,6.32.
However, I could not validate if it builds successfully due to conflict of dependencies between root, STIR, and libparallelproj=2 (conda installations).

Related issues

Fixes #1677

Checklist before requesting a review

• I have performed a self-review of my code
• The code builds and runs on my machine
• documentation/release_XXX.md has been updated with any functionality change (if applicable)

Contribution Notes

Please tick the following:

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in STIR (the Work) under the terms and conditions of the Apache-2.0 License.

[codacy-production]

Up to standards ✅

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[KrisThielemans]

possibly still add a quick line in the release notes ("If CERN ROOT is used, set minimum C++ standard from its ROOT_CXX_STANDARD variable, if it exists."

[KrisThielemans]

Why draft? If the tests work, I'm fine to merge.

[denproc]

I proposed conditional upgrade as the minimum C++17 was set in the snippet below. However, I agree without condition it look more clean, while at config stage it costs nothing to reset it back to version 17.

STIR/CMakeLists.txt

Line 38 in 0d6ab3d

UseCXX(17)

My inital idea for draft was to give me some time to test the build with ROOT installed.
However, it's not that straghtforward as I expected due to the conflict between dependencies.

Let's merge this PR as it shouldn't affect the use cases, where ROOT is not used. Any compatibility problems, when builiding STIR with ROOT installed, could be addressed as separate issue.

[denproc]

Switching back to draft as currently it fails for standalone ROOT install.

[denproc]

Hi @KrisThielemans,

I have updated the PR.
Tested on macOS with ROOT 6.28, 6.30, and 6.32, using both find_package(ROOT) and ROOTSYS + root-config.

Previously, ROOT >= 6.30 was assumed to require C++20. However, pre-built ROOT distributions (including conda) use C++17 by default. I therefore propose using C++17 as the default for ROOT < 6.32.
For custom ROOT builds using a different C++ standard, the user should set CMAKE_CXX_STANDARD manually.

[KrisThielemans]

suggestions for the release notes. I'm fine with the code itself. (Please commit changes with [ci skip])

[KrisThielemans]

Thanks!