Previously, Meson only allowed most directory options to be relative to prefix. This restriction has been now lifted, bringing us in line with Autotools and CMake. It is also useful for platforms like Nix, which install projects into multiple independent prefixes.
As a consequence, get_option might return absolute paths for any directory option, if a directory outside of prefix is passed. This is technically a backwards incompatible change but its effect should be minimal, thanks to widespread use of join_paths/ / operator and pkg-config generator module.