ghc runs preprocessor too much
When an external preprocessor has been specified with -F -pgmF, and then using ghc --make, ghc runs the preprocessor on every .hs file, even those for which compilation is skipped. This seem unnecessary and it slows down recompilation significantly when you have many files.
> -- Lennart
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 6.7 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Driver |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | Unknown |
| Architecture | Unknown |
</details>
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