AI will attack its own buildings when built on player's concrete

[rofl0r]

when the AI puts a building on concrete that was created by the player, its own units will attack the building

[wangds]

The AI can attack its own structure if it was built on top of the player's structure.
As far as I could tell, this doesn't happen to structures placed on top of the player's concrete slabs/walls.

In Structure_Place, the structure that is built over isn't removed, so the AI will still target it.
I think we want to retain the building over structures behaviour, otherwise the AI rebuilding will stall.
Not sure how to handle the case where a structure is partially built over though.

[miniupnp]

It would be useful to have a savegame exhibiting this behaviour

[miniupnp]

I'm moving this to milestone 1.0 as I'm waiting for a way to reproduce the problem. A savegame would be useful.