Use ReaderT instead of DerivingVia because of ghc 8.4

Cabal/src/Distribution/Simple/Build/Monad.hs

@@ -1,8 +1,9 @@
-{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 module Distribution.Simple.Build.Monad
-  ( BuildM (..)
+  ( BuildM (BuildM)
   , runBuildM
   , PreBuildComponentInputs (..)
 
@@ -49,8 +50,14 @@ data PreBuildComponentInputs = PreBuildComponentInputs
   }
 
 -- | Computations carried out in the context of building a component (e.g. @'buildAllExtraSources'@)
-newtype BuildM a = BuildM (PreBuildComponentInputs -> IO a)
-  deriving (Functor, Applicative, Monad, MonadReader PreBuildComponentInputs, MonadIO) via ReaderT PreBuildComponentInputs IO
+newtype BuildM a = BuildM' (ReaderT PreBuildComponentInputs IO a)
+  deriving (Functor, Applicative, Monad, MonadReader PreBuildComponentInputs, MonadIO)
+-- Ideally we'd use deriving via ReaderT PreBuildComponentInputs IO, but ghc 8.4 doesn't support it.
+
+-- | Construct a t'BuildM' action from an IO function on 'PreBuildComponentInputs'.
+pattern BuildM :: (PreBuildComponentInputs -> IO a) -> BuildM a
+pattern BuildM f = BuildM' (ReaderT f)
+{-# COMPLETE BuildM #-}
 
 -- | Run a 'BuildM' action, i.e. a computation in the context of building a component.
 runBuildM :: BuildingWhat -> LocalBuildInfo -> TargetInfo -> BuildM a -> IO a