m4/my_lib_readline: Fix check for additional libraries

MY_LIB_READLINE tries each readline-compatible library with a sequence
of additional libraries, starting with none.  If the check with none
fails, all the others checks reuse the cached result, and also fail.
Broken in commit 1cbda2c7d "client: Rewrite readline configuration",
v4.4.0.

Avoid the unwanted chaching.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>