By default, be lax about dependencies on GHC
There are a number of things which technically depend on GHC (e.g. if ghc changes then Haskell files may be compiled differently, or Cabal packages may be configured differently). However, in practice, having a real dependency on GHC is just a pain: We normally don't want to spend time recompiling other things while we're working on the compiler, and even if we did, GHC will normally decide compilation isn't needed anyway. So by default we use order-only dependencies on GHC, i.e. GHC must exist, but if it's newer than other targets then rebuilding is not necessary.
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