* readline.m4 (gl_FUNC_READLINE): Rewrite the cached part of the

* readline.m4 (gl_FUNC_READLINE): Rewrite the cached part of the
test to be side-effect free by storing the result in the cache
variable gl_cv_lib_readline, and moving the assignment of
LIBREADLINE and LTLIBREADLINE outside the COMMANDS-TO-SET-IT.
From Ralf Wildenhues <Ralf.Wildenhues@gmx.de>.