ghc-toolchain: Match CPP args with configure script

At the moment we need ghc-toolchain to precisely match the output as
provided by the normal configure script. The normal configure script
(FP_HSCPP_CMD_WITH_ARGS) branches on whether we are using clang or gcc
so we match that logic exactly in ghc-toolchain.

The old implementation (which checks if certain flags are supported) is
better but for now we have to match to catch any potential errors in the
configuration.

Ticket: #23720

utils/ghc-toolchain/src/GHC/Toolchain/Tools/Cpp.hs

@@ -4,12 +4,13 @@ module GHC.Toolchain.Tools.Cpp (HsCpp(..), findHsCpp, Cpp(..), findCpp) where
 
 import Control.Monad
 import System.FilePath
+import Data.List(isInfixOf)
 
 import GHC.Toolchain.Prelude
 import GHC.Toolchain.Program
-import GHC.Toolchain.Utils (withTempDir)
 
 import GHC.Toolchain.Tools.Cc
+import GHC.Toolchain.Utils (withTempDir)
 
 newtype Cpp = Cpp { cppProgram :: Program
                     }
@@ -39,10 +40,31 @@ findHsCpp progOpt cc = checking "for Haskell C preprocessor" $ do
 -- Haskell source.
 findHsCppArgs :: Program -> M [String]
 findHsCppArgs cpp = withTempDir $ \dir -> do
+
+  let tmp_c = dir </> "tmp.c"
+  writeFile tmp_c ""
+  (_, stdout0, stderr0) <- readProgram cpp ["-x", "c", tmp_c, "-dM", "-E"]
+
+  if "__clang__" `isInfixOf` stdout0 || "__clang__" `isInfixOf` stderr0
+     then return ["-undef", "-traditional", "-Wno-invalid-pp-token", "-Wno-unicode", "-Wno-trigraphs"]
+     else do
+        (_, stdout1, stderr1) <- readProgram cpp ["-v"]
+        if "gcc" `isInfixOf` stdout1 || "gcc" `isInfixOf` stderr1
+          then return ["-undef", "-traditional"]
+          else do
+            logDebug "Can't recognize your CPP program, you may need to set --with-hs-cpp-flags=FLAGS explicitly"
+            return []
+
+
+{- TODO: We want to just check which flags are accepted rather than branching on which compiler
+         we are using but this does not match what ./configure does (#23720)
+
+         When we retire configure then this more precise logic can be reinstated.
+  withTmpDir $ \dir -> do
   let tmp_h = dir </> "tmp.h"
 
       -- Werror to ensure that unrecognized warnings result in an error
-      checkFlag flag =
+  let checkFlag flag =
           checking ("for "++flag++" support") $ callProgram cpp ["-Werror", flag, tmp_h]
 
       tryFlag flag =
@@ -56,6 +78,7 @@ findHsCppArgs cpp = withTempDir $ \dir -> do
       , tryFlag "-Wno-unicode"
       , tryFlag "-Wno-trigraphs"
       ]
+      -}
 
 ----- C preprocessor -----