cabal new-haddock does not work for autogenerated gi-* bindings

Hello,

I am having some trouble trying to generate docs for the `gi-*` set of libraries I maintain (autogenerated bindings for `gtk` and friends) using `cabal new-haddock`.

The bindings all use `build-type: Custom`, and generate the list of modules to be compiled when configuring the package using a `confHook` (see [here](https://github.com/haskell-gi/haskell-gi/blob/master/lib/Data/GI/CodeGen/CabalHooks.hs)), using a custom `Setup.hs` script. The list of modules to be compiled depends on the system compiling the library, and cannot be known in advance, hence the weird build system.

Despite the somewhat involved `Setup.hs` logic, both `cabal new-build` and `cabal old-haddock` have been working well in practice so far.

But `cabal new-haddock` seems to do nothing:
```
$ git clone https://github.com/haskell-gi/haskell-gi.git
$ cd haskell-gi
$ cabal new-haddock --haddock-for-hackage gi-glib
Build profile: -w ghc-8.6.1 -O1
In order, the following will be built (use -v for more details):
- gi-glib-2.0.18 (file /home/inaki/cvs/haskell-gi/dist-newstyle/build/x86_64-linux/ghc-8.6.1/haskell-gi-0.21.5/cache/build changed)
```
Any advice? I would like to switch to the new workflow, but this issue has been stopping me from doing so.