Qt is not required in xpce, as discussed in a previous topic. xpce just requires a bunch of X libraries, which are admittedly large when compared to small libraries like libyaml, but nowhere near the size of any modern gui framework.
The sizes reported are runtime dependency sizes. If Qt had been a runtime dependency, it would have been counted.
That said, to do anything with xpce you do need some sort of X server, which on most systems is either going to be Xorg or Xwayland. Probably you’d have some sort of desktop environment to go with that. I’m not accounting for any of that.
I am not sure if this choice needs to be made at compile time. For sure, that’s how gentoo would have to do it, being fully source based, but for any distro that uses a cache of pre-built things, it is possible to use a single source package to produce several output packages. Compile time can just build everything, as long as the build output is properly split up in several packages. People depending on SWI-Prolog with a particular feature set can then just pull in the right set of packages to get everything.
This is possible as long as java and python support are things that live in files that can safely be removed while retaining a functioning SWI-Prolog that just lacks these features.
I hope this is possible, because I’m not looking forward to installing swi-prolog-nox-nojava-python.