Building on NetBSD against readline-5.0, I got compilation errors because some files declare readline functions (in a way inconsistent with the readline .h files).
Readline 5.0 was released 4 years ago. I don't think we need to continue to support older versions, particularly when building with newer ones causes compilation errors.
While I was at it, I updated the INSTALL and aclocal.m4.
diff -u for tree