It depends on the thing being built, but the short answer is "yes and no." The non-AI factions build all sorts of stuff. The AI should be building turrets and some other things of that nature (and from my testing they do seem to be). As far as constructing specific units like forcefields... I don't think that they have any logic for that right now, since they didn't in the first game. Guard posts aren't really something they rebuild, either, to my knowledge. But the list of things they build will expand in the future (barracks being one obvious thing).
