Implement native CPP

Proposal/NativeCpp explains why we might want to embed a C-like preprocessor (e.g., hpp) into GHC. This ticket tracks that proposal.

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Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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mentioned in issue #1290

changed weight to 5

added Tfeature request Trac import labels

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Trac field	Value
Blocking	- → #1290

I suggest putting the Proposal/NativeCpp into ghc-proposals on Github so it can follow the new process.

I think this is really more of am implementation detail. As far as I can tell, Proposal/NativeCpp doesn't really propose any user-facing changes, it merely changes the implementation to be more robust and consistent across platforms. As such, I'm not entirely sure it needs to go through the proposals process.

added Pnormal label

For what it's worth, I'm invoking the preprocessor (through -pgmP cpp) without -traditional to get it to honor ifdefs with leading whitespace (to match the surrounding indent). Dependent on how the switch to a Haskell-version of CPP happens, it may or may not be a user-facing change. :)