If you know which ai will build stonehenge, you can reload a previous save and declare war on them. i'm not sure it's 100% but i had a game where the game crashed when i took an enemy city. I reloaded my most recent auto save which was a couple turns before i declared war on them. Apparently i took a turn longer to declare this time because they finished building the oracle 3 turns before me. From this i assume ai's at war sometimes stop building wanders, even if 1 turn from completion.