Rebased from sanders's patch for #3685 with his permission.
This patch allows defining a "clearance" around the obstruction of a building against which new placements are checked. In effect, it prevents the player from building stuff too close to each other, and specifically here it ensures at least an infantry-sized unit can always pass.
This would IMO be a really nice feature to get.
Obviously, down the road, we should probably add some GUI indications of what's happening, but I don't think that's necessary for now.