Let para_play depend on libreadline.

Let para_play depend on libreadline.

While para_client and para_audioc are suitable for non-interactive
use, e.g. in scripts, para_play can only be used interactively,
and its non-readline version is barely usable.

Rather than building a crippled version, teach the build and test
systems to skip para_play if the readline library is not installed.