build: Don't insist on curses for CLI support.

On systems without curses headers/libs, support for interactive
cli tools is deactivated and a message is shown which claims that
readline depends on the curses library. This is not true: readline
uses the termcap functions, but does not link with the termcap or
curses library itself.  Applications must choose an appropriate
library that provides these functions.

This change should enable readline support on systems that have
libtermcap but no curses library.