Link OpenMP with modern CMake

[boxanm]

Simplifies OpenMP import in CMakeLists.txt.
From https://cliutils.gitlab.io/modern-cmake/chapters/packages/OpenMP.html:

OpenMP support was drastically improved in CMake 3.9+. The Modern(TM) way to add OpenMP to a target is:

Checklist:

Code related

• I have made corresponding changes to the documentation
  (i.e.: function, class, script header, README.md)
• I have commented hard-to-understand code
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
  (Check README.md #Contributing
  for local testing procedure using libpointmatcher-build-system)

PR creation related

• My pull request base ref branch is set to the develop branch
  (the build-system won't be triggered otherwise)
• My pull request branch is up-to-date with the develop branch
  (the build-system will reject it otherwise)

PR description related

• I have included a quick summary of the changes
• I have indicated the related issue's id with # <issue-id> if changes are of type fix
• I have included a high-level list of changes and their corresponding type
  • Types: feat fix docs style refactor perf test build ci chore revert
  • Breaking changes: <type>!
  • Reference:
    see commit_msg_reference.md
    in the repository root for details

---

Note for repository admins

Release PR related

• Only repository admins have the privilege to push/merge on the default branch (ie: master)
  and the release branch.
• Keep PR in draft mode until all the release reviewers are ready to push the release.
• Once a PR from release -> master branch is created (not in draft mode),
  • it triggers the build-system test
  • (in-progress) and it triggers the semantic release automation

[sonarqubecloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud