Support for GHC version (range) specific flags.

[merijn]

When using -Werror on CI, the fact that warning flags change over time can lead to issues where code cannot be warning free across all supported versions. A solution is to toggle of certain warnings for specific GHC versions, but currently there is no way to specify --local-ghc-options for only some GHC versions.

The missing-methods warning is currently special-cased via cfgErrorMissingMethods, it would be nice if we could generalise this to specify "GHC flags per version range".

[phadej]

So far all per-GHC options where configured like:

tests: >= 9.0

-- or
werror: >=9.0

i.e. we have a very specific thing and we set a compiler version when it's enabled.

With things like ghc-options it's hard(er) as I don't know how we would configure them.

if impl(ghc >=9.0)
  local-ghc-options: ...

is one idea, but I don't know how to elegantly implement it.

I'm fine with adding werror option. The more general approach: feel free to try, but do not be offended if I wont merge the patch.

[phadej]

The haskell/cabal#7783 once released will force us to think about this, especially

if impl(ghc >=9.0)
  local-ghc-options: ...

I might or might not try stuff before hand.