GNU readline support for the unix port was removed in
acaa30b6046d449f5f58a8f02c83459702759df7 and in
5e83a75c78dc8c370b25e7ee669295854ea45130, so it's also no longer supported
in the windows port.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.