LLVM’s build system changed from autoconf to CMake many years ago. I couldn’t find specific documentation around the migration, but Building LLVM with CMake — LLVM 17.0.0git documentation is a general overview of LLVM’s CMake build system. libcxx/src/CMakeLists.txt and libcxxabi/src/CMakeLists.txt should be equivalent to the Makefiles you’re describing (I think … I never worked with the old build system so I can’t say for sure), and you’ll have to translate your changes to CMake.