Add a systemd cabal flag

[erikd]

This is needed because neither MacOS nor Windows have systemd but it is desirable on Linux.

Changelog

- description: |
   Add a systemd cabal flag
# uncomment types applicable to the change:
  type: feature        # introduces a new feature

Context

Manual testing:

> cabal build all --dry-run --flag=-systemd 
Up to date
> jq < dist-newstyle/cache/plan.json | grep systemd
        "systemd": false,
        ....

> cabal build all --dry-run --flag=+systemd 
Resolving dependencies...
....

> jq < dist-newstyle/cache/plan.json | grep systemd
        "systemd": true,
         ....
> cabal build all --dry-run 
Resolving dependencies...
...
> jq < dist-newstyle/cache/plan.json | grep systemd
        "systemd": true,
         ...

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• The change log section in the PR description has been filled in
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • round trip tests
  • integration tests
    See Running tests for more details
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• The changelog section in the PR is updated to describe the change
• Self-reviewed the diff

[erikd]

Need to remove this to avoid warnings.

Was not able to conditionally disable the warning.

[newhoggy]

Do you get systemd support just by adding the package? Or is there some Haskell code incoming. Also, why does the CLI need systemd support?

[angerman]

I think we might have had some miscommunication here. Building cabal build -j cardano-cli in the cardano-node repo. will complain about the libsystemd-journal dependency; that is however more an issue of building Cardano-cli from the node repo than a Cardano-cli issue. I'm sorry I sent @erikd down this hole.