When building a binary or library, specifying the rpath, i.e.
-Wl,rpath,<path/to/lib>
tells the linker where to find the required library at runtime of the binary.
What is the UNIX philosphy regarding absolute and relative paths here? Is it better to use an absolute path so the lib can be found from everywhere? Or is it better to make it relative so copying an entire directory or renaming a higher level path won't render the binary unusable?
Using $ORIGIN is usually the preferred way of building binaries. For libraries I like to put in the absolute path, because otherwise you won't be able to link to the library. A symbolic link will change the $ORIGIN to point to the path of the link and not of the link target.
What is the UNIX philosphy regarding absolute and relative paths here?
Using relative path makes an executable that only works when invoked from a particular directory, which is almost never what you want. E.g. if the executable is in /app/foo/bin/exe and has DT_RUNPATH of lib/, and a dependent library is in /app/foo/lib/libfoo.so, then the exe would only run when invoked from /app/foo, and not when invoked from any other directory.
Using absolute path is much better: you can do cd /tmp; /app/foo/bin/exe and have the executable still work. This is still however less than ideal: you can't easily have multiple versions of the binary (important during development), and you dictate to end-users where they must install the package.
On systems that support $ORIGIN, using DT_RUNPATH of $ORIGIN/../lib would give you an executable what works when installed anywhere and invoked from any directory, so long as relative paths to bin/ and lib/ are preserved.