m4: fix readline macro

Beniamino Galvani requested to merge bg/m4-readline into main

The macro always overwrites LIBS and the result is that every binary links against libreadline.

Instead, save the library to READLINE_LIBS.

See also: 94274c6f ('build: fix wrongly linking against libreadline in all applications')

Fixes: af360238 ('m4/ax_lib_readline.m4: Update after running aclocal')

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