If your project strictly requires cctools 6.5, use the Apple open-source tarball and compile it within a macOS 10.9 virtual machine for best compatibility. For nearly all other cases, upgrade to a modern toolchain.
This should display the version of the clang compiler. cctools 6.5 download
Unlike standard GNU tools, cctools handles Apple’s specific binary formats (Mach-O) and "Fat" (Universal) binaries. If your project strictly requires cctools 6