Fairy-Max (which ships with the XBoard package) can play variant chess. Is there any other XBoard-compatible engine that also plays variant chess (and runs on Linux or *BSD)?
The XBoard Alien GUI is reputedly better suited to playing chess variants than the standard XBoard is, but that's just an observation. There are many engines that play only a few variants.
Here's a list of engines (other than Fairy-Max) that have been noted as playing a large selection of variants and working with XBoard (Linux environment being implicit):
add Fairy Stockfish to that list... a recent development