Cannot find generated header files when building `unix-time` with Cabal 2.2

I have been trying to build hoogle using `cabal new-build hoogle`. It fails on building `unix-time` with `fatal error: config.h: No such file or directory`.

This is likely due to a missing `include-dirs: cbits` in the cabal file for `unix-time`. This was determined by a successful build of `kazu-yamamoto/unix-time` release `v0.3.7` by including that missing line. The build succeeded with `cabal new-build`.

This was found using cabal-install 2.2.0.0, ghc 8.0.2, WSL Ubuntu 16.04.

Start by reproducing the failure with cabal new-build hoogle using cabal-install 2.2.0.0, GHC 8.0.2, and WSL Ubuntu 16.04. Inspect the unix-time Cabal file and generated config.h handling; done means the dependency builds successfully without the missing-header error.

Written by the indexing model from the issue text.