eSpeak (a compact open source software speech synthesizer)

eSpeak produces good quality English speech. It uses a different synthesis
method from other open source text to speech (TTS) engines, and sounds
quite different. It's perhaps not as natural or "smooth", but some find
the articulation clearer and easier to listen to for long periods. It
can run as a command line program to speak text from a file or from stdin.

To build without portaudio support, set WAVONLY=yes in the script's
environment.

If you get errors like this:

bt_audio_service_open: connect() failed: Connection refused (111)

...either ignore them (if espeak produces audio anyway), or comment out
the 'pcm.headset' and 'ctl.headset' stanzas in /etc/asound.conf. This
should only happen if your Slackware system was upgraded from an older
version.
