Haskell-only "headers" stil require C preprocessor

**Describe the bug**

Some packages the `includes:` stanza just for CPP'ing Haskell, e.g. `bifunctors-5.5.5`. When compiling with GHCJS (or CLaSH) we might not want to not provide a C compiler, because C is non-trivial (or impossible) to compile for these platforms. However Cabal will want to use a C compiler to ensure the header exists.

**To Reproduce**
`cabal configure --with-gcc=false` `bifunctors-5.5.5`. Command fails with:
```
...
Setup: Missing dependency on a foreign library:
* Missing (or bad) header file: bifunctors-common.h
...
```

**Expected behavior**

If we wish to better support CPP for Haskell (and perhaps this is too jank and doesn't warrant it) I see two basic options options:

1. Use GHC if CC doesn't exist probe for headers and cross fingers.
2. In the Cabal file distinguish the language the included file is for.

1 is the quite quick and dirty, 2 is the quite overwrought. There might be decent designs between?

CC @angerman