fix: replace CMAKE_SOURCE_DIR with CMAKE_CURRENT_SOURCE_DIR

[marfvr]

This PR replaces occurrences of CMAKE_SOURCE_DIR in the CMake configuration file with CMAKE_CURRENT_SOURCE_DIR. This change would allow the project to be usable as a CMake dependency for other CMake projects.

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[nikola-matic]

Hi @marfvr, OSX and Arch builds are failing. Are you planning on continuing work on this?

[marfvr]

Hi @nikola-matic, thank you for your reply.

Yes, I am willing to work on this PR (not immediately, though), as long as it is considered a good change proposal for the project.
Anyway, any of the maintainers/contributors is free to continue from where I left off.

[cameel]

We already have an old pending PR for this: #15539. We should close one of them.

[cameel]

Here's the comment I made the last time @matheusaaguiar asked me about that other PR. I guess I should have just posted it there. It's also still relevant here:

Is this change even correct? We use that value e.g. to find the deps/ directory and that's at the top-level. If I understand correctly, CMAKE_CURRENT_SOURCE_DIR is the dir containing the current CMakeFiles.txt so it will be the wrong one unless we're at the top level. I think it passes CI only because we currently don't invoke any of these changed .cmake files from other CMakeFiles.txt. If we did, this change would probably break things. Though I'd have to test this locally to make sure. I'm basing this only on a quick search.

[clonker]

@cameel you are right with your comment, another one is PROJECT_SOURCE_DIR. However, in this case, the changes should be correct as everything is include(...)-ed. Which doesn't change the CMake directory. It's different from add_subdirectory in that regard. The test failures look unrelated at first glance. Still I am unsure whether we actually want this change, we might rather want to use PROJECT_SOURCE_DIR, which also works if the project is included in other CMake projects as dependency and is more idiomatic in what it refers to.