If the library was instead giving me a .c and a .h, I would simply need to make that CMakeLists.txt, or a Makefile, or whatever build system I use.

Now, the library gives me a .h and a `#define XXX_IMPL`. So to avoid making the mistake of defining XXX_IMPL more than once, I'll have to make a .c myself for each file of the library, and then I still need to write the CMakeLists.txt/Makefile/whatever.

This is just wasting my time for no good reasons.

I linked the imgui repository because that's what they do, they give you the .c/.h and you decide how to build it. You just have to copy/paste, no time wasted.