Rsynth is a speech synthesis program that runs on Unix-like systems. It's public domain software as far as I know, because it's based on public domain code according to the README and lack of a license. It's relatively small compared to other systems (only about 5k LOC).
BSD ports/package systems have rsynth.
Version 2.0 sources can be found here:
http://www.speech.cs.cmu.edu/comp.speech/Section5/Synth/rsynth.html