The ability to configure itcl with threads enabled, but not to attempt to configure itk with threads, might be useful.
It's my understanding that Tk (and I presume iTk as well) are not thread safe under Unix X Window system based windowing. If that's the case (at least for Solaris) then have the ability to configure one without the other would be useful. Or, perhaps, having the --enable-threads trigger the script looking at how tcl and tk were configured, and only configuring itk thread support if tk was configured with threads? I'm uncertain what the best approach might be.