Anon is right, it's called CSG substract. And also, it is evil. I don't remember the exact reason why it was evil (I haven't done any mapping for quite a while and I forgot a lot of the theory), but generally on every mapping/modding forum everyone would tell you not to use it.
Drawing floors, walls, and ceilings by hand, no matter how painful may it seem compared to drawing just the inside and hitting the magic button, is the way to go. If you'd be modelling a wall that is visible from both sides (for example a building in an outdoor area which you can enter), you're going to put a little more effort to properly shaping the brushes at the corners anyway, as to avoid either overdraw or texture-caulk z-fighting in the editor (which looks ugly).
It's a shame that building GtkRadiant on linux is virtually impossible these days. Code rot - the project is like, dead. I've once managed to get it running on some another machine, but it broke since then.