osxrelocator: Fix RPATHs from libexec executables
There's an error being carried over from before !1485 (merged): the depth calculation needs to be done folder-to-folder, so it was always off by one. This error had been papered over by preserving the /lib/ folder in the dylib ID, but since !1485 (merged) it was revealed by breaking running the GStreamer executables in libexec.
This commit fixes it by properly doing the depth calculation. I've also
renamed the lib_prefix
parameter as it is in fact the install prefix.
Fixes #499 (closed)
PS: This MR integrates another commit because I was not able to bootstrap macOS correctly, I tracked it down to !1587 (merged).