C pre-processor failed in ide-backend

[lukehoersten]

This happens on the io-streams codebase. I can move the issue to ide-backend project if we think it's an up-stream problem. Wanted to check w/ @chrisdone here first though.

<unknown>:(0,0)-(0,0): 
ide-backend-server: phase `C pre-processor' failed (exitcode = 1)