The cheap way to do it is to open up the object editor, go to the building you want to rotate and go to where it says Is A Building and change that to false. Then go back into the map view, select the building and hold CTRL and spin it. When you have the angle you need, go back into the object view and make it a building again.
The only other way I can think of is through triggers. If the above doesn't work, just tell me and I'll make a quick trigger for you.