There is a lof of handling of the third party libraries in the top level CMakeLists.txt.

The top level CMakeLists.txt should optimally only have a add_subdirectory(thirdparty). In the thirdparty directory there should be a CMakeLists.txt which contains

add_subdirectory(tinyscheme)
add_subdirectory(dxflib)

Then respective thirdparty directories should have a CMakeLists.txt with all settings for that library in there. And be careful about PRIVATE and PUBLIC on what is exported. All code in the toplevel CMakeLists.txt should move to respective libraries CMakeLists.txt.

Both thirdparty libraries should then create a static library like add_library(<library name> STATIC) and then be linked in every place it is needed.

I also question if we need to check for (and use) other DXF libs if we have it bundled into the code. Any suggestions or reasons to keep looking for it?

Maybe I'm rude, but I would also suggest to do these changes first, then rebase all PR's involving CMake work for the thirdparty libraries onto the new structure.