ghc recompilation check doesn't take into account headers directly included by CApiFFI

Continuing from #14756, given the following simple module

{-# LANGUAGE CPP, CApiFFI #-}

#include "Bar.hs.inc"

module Foo where

foreign import capi unsafe "cbits.h foo" c_foo :: Int -> IO ()

After having compiled ghc -c Foo.hs successfully, and second invocation of ghc -c Foo.hs

• ...will properly recompile when Bar.hs.inc changes (or any of its recursive #includes!) (good!)
• ...will fail to detect when cbits.h changes, and thus claim compilation IS NOT required (BAD!)

Ideally, we should always run CPP for the CApiFFI-included, in order to detect when any recursive #includes inside cbits.h change, and use a recorded checksum to detect whether recompilation of the C wrapper (and thus re-linking of the haskell module; which currently forces resynthesizing the object files) is necessary.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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