If you know that the executables will be in a certain directory, such that ../lib relative to that is where the libraries are found, then you just make the rpat

If you know that the executables will be in a certain directory, such that ../lib relative to that is where the libraries are found, then you just make the rpath '$ORIGIN/../lib'. That's it; no guesswork. Build systems that make it a huge paint to calculate elaborate rpath, e.g. knowing what the sysroot will translate to on the actual target system or whatever, are just doing counterproductive nonsense. By the way, I seem to recall that Yocto uses chrpath in order to get its own tools to reference its own copy of glibc that it provides (for itself, in the build environment).