Come to a hard-stop if user wants libreadline, but it's not available.

Default is to check and include, or not include if not available, but
if user option is "--with-libreadline" then the library and header files
must be available, otherwise ./configure comes to a hard-stop.
Merge pull request #1406 from JoesCat/check_headers_and_libraries_readline

Come to a hard-stop if user wants libreadline, but it's not available.