--with-ispell description elsewhere in the doc seems to confirm this. But in my case it was clearly necessary to set --with-ispell correctly before spell-checking would work. Maybe the configure script only looks in certain places, and maybe only for the name ispell and not aspell? I didn't check on this.
Configure looks in your $PATH. Well, in theory, at least.
If you installed aspell in some random place in your filesystem, you obviously can't expect configure to find it.
Now, there is a small bug in the configure script where the last component in PATH is not searched. So configure won't find anything installed in the last directory listed in PATH, at this time.
