Add back support for relocatable packages

[mrexodia]

Your checklist for this pull request

• I've documented or updated the documentation of every API function and struct this PR changes.
• I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

The previous PR #2134 not only 'allowed' an absolute CMAKE_INSTALL_LIBDIR/CMAKE_INSTALL_INCLUDEDIR, it also broke support for relocatable packages if the path was not absolute.

These changes should also be cherry-picked in v5.

Discussion: NixOS/nixpkgs#144170

[Rot127]

These changes should also be cherry-picked in v5.

Could you please open a PR with it?
We might even get it into the v5.0.2 path release today or tomorrow.

[Rot127]

In general looks good to me. Can you tell me how quickly this becomes "tested" via NixOS?

[Rot127]

@kabeor Why is the CI not running here?

[mrexodia]

In general looks good to me. Can you tell me how quickly this becomes "tested" via NixOS?

No idea, I do not use NixOS. They are doing something different that is specifically recommended against by CMake's documentation. This patch goes back to the behavior before #2134, unless those paths are absolute (as will be the case with NixOS). It's silly this workaround is required, but such is life 🤷🏻‍♂️

[kabeor]

Really wired, may be affected by github downtime? Can we commit something or open a new pr to see if CI come back?

[mrexodia]

This one also doesn't run CI: #2447

[XVilka]

If CI doesn't run, it might be the sign of some error in the Actions YAML file. Check "Actions" log of the repository.
Note also an error of using outdated NodeJS runner: https://github.com/capstone-engine/capstone/actions/runs/10419892239

[Rot127]

@XVilka The yaml files are fine. The CI also runs for the other PRs as well.
@mrexodia Can you rebase this one? The labeler was at least working on the v5 PR. Maybe it is indeed a Github outage problem.

[mrexodia]

Done!

[Rot127]

For me all of the checks are green now.
@kabeor Please take a look.

[kabeor]

Looks good to me, thanks.