Updated CMake files to use CMAKE_CURRENT_SOURCE_DIR

[fagsctvg]

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[cameel]

Here's the comment I made the last time @matheusaaguiar asked me about this PR. I guess I should have just posted it here. Anyway:

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.