This icon should be on every neutral building. When you decide to convert the structure, a menu would appear and you select what to build.
Buildings that do not belong to any alliances, such as huts and houses, should be transformed into bunkers. Plus, trees and unarmed towers may be altered so that a turret/weapon is mounted at the top of the tower/tree.
Also, sky scrapers can be changed to command centers and/or command relay posts.

