Simplify rpath setting

[probonopd]

Should we simplify rpath setting by delegating the job for a whole directory using a patchelf option to make the rpath relative under a specified root directory?

NixOS/patchelf#118

It would probably mean that we also need to resolve (and copy in) all the dependent libraries in advance.

Might speed up things, but according to my mantra "never touch a running system" it will only get implemented if someone (not me) finds this valuable enough to do the work (and testing).