.hsc files are not reprocessed when CPP definitions change

In regular `.hs` files, changing the result of a `#ifdef VAR` by adding or removing a `-DVAR` causes recompilation. However, this is not the case for `.hsc` files: the `.hsc` files are not reprocessed when when their `#ifdef` results change. A simple solution might be to regenerate the `FOO.hs` from `FOO.hsc` on each build, and then rebuild `FOO.hs` when the new version differs from the old version. But this would slow down the common case in order to fix the special case.

I uploaded a simple, concrete example at https://github.com/ntc2/cabal-hsc2hs-bug and have pasted the README below:
# Illustrate hsc2hs+cabal bug
## Problem

The `HSC.hsc` file does not get reprocessed when the CPP definitions
change, only when the `HSC.hsc` file itself is changed. On the other
hand, the regular Haskell `HS.hs` file does get reprocessed when the CPP
definitions change.
## Cabal and GHC versions

```
$ cabal --version
cabal-install version 1.22.6.0
using version 1.22.2.0 of the Cabal library

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.10.2
```
## Setup

```
$ cabal sandbox init
```
## Source files

```
$ cat bug/Main.hs
module Main where

import HS
import HSC

main = do
putStrLn $ "hsTest = " ++ show hsTest
putStrLn $ "hscTest = " ++ show hscTest

$ cat src/HS.hs
{-# LANGUAGE CPP #-}
module HS where

hsTest :: Bool
#ifdef CPP_FLAG
hsTest = True
#else
hsTest = False
#endif

$ cat src/HSC.hsc
{-# LANGUAGE CPP #-}
module HSC where

hscTest :: Bool
#ifdef CPP_FLAG
hscTest = True
#else
hscTest = False
#endif

$ cat cabal-hsc2hs-bug.cabal
name: cabal-hsc2hs-bug
version: 0.1.0.0
license: BSD3
license-file: LICENSE
author: Nathan Collins
maintainer: nathan.collins@gmail.com
build-type: Simple
cabal-version: >=1.10

flag cabal_flag
description: a Cabal flag
default: True
manual: True

library
exposed-modules: HS, HSC
other-extensions: CPP
build-depends: base
hs-source-dirs: src
build-tools: hsc2hs
default-language: Haskell2010

if flag(cabal_flag)
cpp-options: -DCPP_FLAG
cc-options: -DCPP_FLAG

executable bug
main-is: Main.hs
hs-source-dirs: bug
build-depends: base, cabal-hsc2hs-bug
default-language: Haskell2010
```
## Observed, buggy behavior

```
$ for prefix in "+" "-"; do \
cabal configure --flags="${prefix}cabal_flag" && \
cabal build -j1 && \
cabal copy \
&& .cabal-sandbox/bin/bug; \
done
[...]
hsTest = True
hscTest = True
[...]
hsTest = False
hscTest = True
```

Notice that `hscTest` is always `True`. If you instead configure with
the flag disabled the first time, then `hscTest` is always `False`.
## Expected behavior

By touching the `HSC.hsc` file first, we can force a regeneration of
the `HSC.hs` file:

```
$ for prefix in "+" "-"; do \
touch src/HSC.hsc && \
cabal configure --flags="${prefix}cabal_flag" && \
cabal build -j1 && \
cabal copy \
&& .cabal-sandbox/bin/bug; \
done
[...]
hsTest = True
hscTest = True
[...]
hsTest = False
hscTest = False
```

See file `FULL_OUTPUT.md` if interested in full output.