Adds another post install hook that loops over the install prefix, looking for shared libraries type of ELF files, and sets the soname to their own absolute paths.
The idea being, whenever somebody links against those libraries, the linker copies the soname (which is the absolute path to the library) as a "needed" library, so that at runtime the dynamic loader realizes the needed library is a path which should be loaded directly without searching.
As a result:
1. rpaths are not used for the fixed/static list of needed libraries in the dynamic section (only for _actually_ dynamically loaded libraries through `dlopen`), which largely solves the issue that Spack's rpaths are a heuristic (`<prefix>/lib` and `<prefix>/lib64` might not be where libraries really are...)
2. improved startup times (no library search required)
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