Add a check for shelltest flag

[basile-henry]

Without this additional flag check the shelltest build tool is always a dependency.
This was an issue for me when using nix to build ghc-mod via the cabal2nix utility as it then depends on shelltest regardless of the value of the flag.

[DanielG]

LGTM. Just curious -- why is it a problem if it depends on shelltest?

[basile-henry]

I think the main issue with nix and the build-tools field is that nix has no idea how to build shelltest as a dependency. My understanding is that it is an executable provided by the shelltestrunner package. Maybe nix would work as expected if the build-tools field was set to shelltestrunner instead of shelltest. I have not tried this.

I am not really familiar with the build-tools field in cabal and I am fairly new to the nix ecosystem. Since you already had a flag choosing whether the shelltest target was buildable or not I thought it made sense to disable shelltest as dependency as well.

I have just had a quick look at what is expected from the build-tools field. From issue haskell/cabal#3708 I think a better way to add the dependency might be:

tool-depends: shelltestrunner:shelltest

Just tested this new field quickly, I don't think cabal2nix supports it. 😞

[DanielG]

build-tool-depends was only added in Cabal-2 so that would be too recent anyways.