In rP7775, UnitAI was rewritten, among the changes made was that a foundation being finished sent a message.
Since at that time there was no way of telling which units took part in the construction, a global message was handled in UnitAI, along with a ToDo that that is expensive and unnecessary.
rP23449 Introduced a function to get the builders for both a foundation and a repairable target, which means that the messages can now be sent targeted. Since it was discussed on IRC and regarding D2704 that messages ought to be sent to themselves only, this patch queries the UnitAI's of the builders to directly call the ConstructionFinished function.