unrecognized field abi warning when cabal installing nixy builds branch

```
~/D/r/cabal-nixy-build (nix-local-build|) $ cabal install cabal/
Resolving dependencies...
Configuring Cabal-1.24.0.0...
Building Cabal-1.24.0.0...
Warning:
/var/folders/py/wgp_hj9d2rl3cx48yym_ynj00000gn/T/pkgConf-Cabal-1.24.058569.0:
Unrecognized field abi on line 68
Installed Cabal-1.24.0.0
Updating documentation index
/Users/carter/.cabal/share-x86_64-osx-ghc-7.10.3/doc/x86_64-osx-ghc-7.10.3/index.html

```

Reproduce the warning with the reported `cabal install cabal/` command and inspect the generated `pkgConf-Cabal-1.24.0.0` file, especially line 68 containing the `abi` field. Trace how Cabal reads that package configuration and determine the expected behavior when the field is encountered; done means the installation no longer emits an unrecognized-field warning.

Written by the indexing model from the issue text.