You are right (I did not think to check before I posted).
I had been maintaining two sets of Code::Blocks' source code on my hard-drive: one to make patches against, and one to compile plugins against. Unfortunately, while I had kept the one for patches up to date, I forgot to update the one for plugins. (It was all the way back at r7460!)
Updated the second set to r7597 -> everything works as expected (although, it seems I will have to update the API my plugin uses).
Thank you.