You are conflating the C ABI with the C++ ABI. The C ABI is stable, so linking to SDL works fine. The C++ ABI is actually relatively stable as far as I can tell, but ABI breaking changes are announced when they are released by compiler vendors.
I am intentionally conflating the C ABI with the C++ ABI because the C++ ABI is only relatively stable so if you want to write software that doesn't require you to rebuild everything each time you update a compiler then you push your code to use more C ABIs rather than more advanced C++isms. This is exactly what SDL has done as the headers are C compatible but it has plenty of C++ in the direct3d parts.
This was originally about grouping single file header libraries into compilation units that wouldn't need to be changed often. I'm not sure what your point here is, since most would have C functions for their interface or use templates. Even if you were using a C++ ABI it would be trivial to recompile. Qt is the furthest thing from a single file library.
