Looks like the discussion isn't over yet, so I'll reopen the ticket.
What do you suggest that it check for? `rst2html` is the part of docutils Geany uses, and that is already tested for.
It's checking that the file exists, but doesn't check that it works.
Among other things, configure checks to make sure the c compiler works, and that some functions can be found, and work, so checking for a *working* rst2html might not be inappropriate.