Hello Ralf, I found it! It was an utterly perversely subtle typographical error:
In my src/Makefile.am: tuxmathserver_SOURCES = servermain.c \ server.c \ mathcards.c \ throttle.c \ . options.c ^ | | A stray '.' got inserted in the file list, which lead to the entire src directory getting dragged into the tarball. I don't quite understand the operation of automake and make well enough to see how this lead to the VPATH build failure (and hence failure of make distcheck), but at any rate removing the '.' fixed the problem. Anyway, thanks a million for looking over tuxmath's autotools files. David Bruce