This is not seen on all distributions (mostly on Debian-based ones in my
experience), but some distributions install libraries in a second-level
directory under prefix (e.g. lib/x86_64-linux-gnu/ instead of lib/ or lib64/)
whereas our prefix-guessing code for relocatable builds harcoded moving up from
1 level.

This new heuristic will assume that if the leaf directory is neither bin/ nor
starting with `lib`, then it's likely a multiarch folder and we must move up
once more to find the prefix folder.

This should also fix the problem encountered by Bruno for the current work on a
potential official AppImage.