Cabal new-build doesn't set data directory environment variables during Setup

`cabal-install` appears not to set the environment variables allowing `Setup.hs` to find data directories for inplace-installed `setup-depends`. This can cause `Setup.hs` to fail for some packages.

For instance, something like the following should demonstrate this:
```
$ cabal unpack gio-0.13.3.1
$ cabal unpack gtk2hs-buildtools-0.13.3.0
$ echo "packages: ./gio-0.13.3.1, ./gtk2hs-buildtools-0.13.3.0" > cabal.project
$ cabal new-build all
```