This is hopefully a temporary setback since we want to consider the
introspected API as a core feature (otherwise we'd end up with third
party builds deactivating the feature, and get a very random experience
depending on where you got GIMP) hence don't want it to be optional
(note that I haven't added a configure option).
Anyway for now, it seems that cross-building introspected data is not
really supported (or I'm still figuring it out).