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:

Rsynth Say Frontend

Category Speech Synthesis