build: suggest -DENABLE_IPC=OFF when missing capnp #33315

1. 
   Sjors commented at 6:42 am on September 5, 2025: member

   Placeholder for the closed PR #33290.

   Currently being worked on in https://github.com/bitcoin-core/libmultiprocess/pull/205

   After which we’ll need a subtree update.

2. 
   fanquake commented at 9:30 am on September 5, 2025: member

   Thanks, but I don’t think we need a separate issue to track the subtree update, which wont be forgotten.
3. fanquake closed this on Sep 5, 2025

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4. 
   ryanofsky commented at 8:52 pm on September 5, 2025: contributor

   Subtree update is #33322

Contributors
Sjors fanquake ryanofsky