client: Rewrite readline configuration

client: Rewrite readline configuration

AX_LIB_READLINE tries to cope with systems where readline lacks
history support, or lacks headers, or needs headers included in
unorthodox ways.  It puts six HAVE_ macros into config.h, and its
usage example takes 24 lines of code just to include two headers.

Way too complicated for my taste.  Replace with new MY_LIB_READLINE,
which succeeds only when you have a sane readline, and then defines
*one* macro: HAVE_LIBREADLINE.

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