Implement SYSTEM handler for cmake dependencies.

[luigifcruz]

The current dependency handler for CMake configuration files doesn't handle including paths that were marked as SYSTEM. This can lead to header conflicts on certain occasions and ultimately hard-to-debug compilation failures.

In my particular case, my program compilation fails when importing anything built with the NVIDIA RAPIDS CMake package. Their system depends on a newer version of libcudacxx that needs to be evaluated after the system includes of CUDA, which contains an older version of libcudacxx. This is achieved by marking libcudacxx as INTERFACE_INCLUDE_DIRECTORIES (-I) and CUDA as INTERFACE_SYSTEM_INCLUDE_DIRECTORIES (-isystem). Since Meson ignores the SYSTEM flags, it just includes everything with -I and the conflict begins.

[dcbaker]

Failures looks relevant. It would also be good to add something to one of the existing tests to cover this.

[luigifcruz]

@dcbaker Are you sure about the title change? Looks like the handlers for dependency and subprojects are different. This PR adds support to import ***.cmake packages while #6199 adds support for subprojects.

[dcbaker]

You are right, I just looked at the traceparser and thought that was subprojects

[eli-schwartz]

Revert manual merge line deletion.

Please always use git commit --fixup followed by git rebase -i --autosquash (or just git commit --amend if it is the most recent commit) to fix issues in previous commits.

In general, each commit should represent a discrete change proposal or step in the process of achieving the goal of a PR.

[luigifcruz]

@eli-schwartz Yes, I'll just check if this will pass CI and then cleanup for merge.

[luigifcruz]

All tests appear to be passing now. I'll add a test for SYSTEM includes shortly.

[eli-schwartz]

This doesn't include any tests for SYSTEM so all we actually know is that it doesn't regress currently tested scenarios.

Would it be possible to add some simple mockup of the original issue? e.g. one of the tests in test cases/cmake/ sets up dependency(xxx, method: 'cmake', cmake_module_path: 'cmake') to "Try to find a dependency with a custom CMake module". It doesn't have to be a full-blown built project, just whatever minimal interface it takes to to fail when -isystem is missing.

(Perhaps some trick with #include_next?)