Allow cabal to build a cabal package that only has a main c file.

[angerman]

Cabal currently expects there to be at least one Haskell module, which prevents the following cabal file to build:

cabal-version: >=2.1
Name: unlit
Version: 0.1
Copyright: XXX
License: BSD3
Author: XXX
Maintainer: XXX
Synopsis: Literate program filter
Description: XXX
Category: Development
build-type: Simple

Executable unlit
    Default-Language: Haskell2010
    Main-Is: unlit.c
    C-Sources: unlit.c

[hvr]

Here's btw an old related issue: #497

[23Skidoo]

This should already work actually, see #1080 and PackageTests/CMain.

[angerman]

@23Skidoo no. That test, has a crucial ingredient: the Bar.hs module.

[23Skidoo]

Ah, OK. Shouldn't be that hard to fix, I think.

[hvr]

@23Skidoo ...and which libraries shall the C-only component be linked with? which headers shall be in -I? I'm specifically thinking about the GHC RTS; shall it be available to a main-is: main.c w/o any haskell modules?

[23Skidoo]

@hvr

I'm specifically thinking about the GHC RTS; shall it be available to a main-is: main.c w/o any haskell modules?

I'd say that it shouldn't, unless you have requested it explicitly with something like build-depends: rts. Though I'm not sure that that's how ghc -c foo.c works right now.