build: Set TEXMFVAR to ensure TeX cache files can be written to build dir * doc/module.mk (TEXMFVAR): New variable, export to environment. (TEXINPUTS): Include $(abs_top_builddir)/doc/.texmf-var// in the list. (doc-clean): Delete doc/.texmf-var.

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");