Make 'cabal haddock' always define __HADDOCK__ when preprocessing.

This is useful e.g. for "writing documentation that links to module A without
explicitly qualifying everything, where A is not directly imported." (see the
discussion in haskell#926)

Fixes haskell#1237.

Cabal/Distribution/Simple/Haddock.hs

@@ -211,14 +211,14 @@ haddock pkg_descr lbi suffixes flags = do
             let bi = libBuildInfo lib
             libArgs  <- fromLibrary verbosity tmp lbi lib clbi htmlTemplate
             libArgs' <- prepareSources verbosity tmp
-                          lbi isVersion2 bi (commonArgs `mappend` libArgs)
+                          lbi bi (commonArgs `mappend` libArgs)
             runHaddock verbosity keepTempFiles confHaddock libArgs'
         CExe exe -> when (flag haddockExecutables) $ do
           withTempDirectory verbosity keepTempFiles (buildDir lbi) "tmp" $ \tmp -> do
             let bi = buildInfo exe
             exeArgs  <- fromExecutable verbosity tmp lbi exe clbi htmlTemplate
             exeArgs' <- prepareSources verbosity tmp
-                          lbi isVersion2 bi (commonArgs `mappend` exeArgs)
+                          lbi bi (commonArgs `mappend` exeArgs)
             runHaddock verbosity keepTempFiles confHaddock exeArgs'
         _ -> return ()
 
@@ -236,11 +236,10 @@ haddock pkg_descr lbi suffixes flags = do
 prepareSources :: Verbosity
                   -> FilePath
                   -> LocalBuildInfo
-                  -> Bool            -- haddock >= 2.0
                   -> BuildInfo
                   -> HaddockArgs
                   -> IO HaddockArgs
-prepareSources verbosity tmp lbi isVersion2 bi args@HaddockArgs{argTargets=files} =
+prepareSources verbosity tmp lbi bi args@HaddockArgs{argTargets=files} =
               mapM (mockPP tmp) files >>= \targets -> return args {argTargets=targets}
           where
             mockPP pref file = do
@@ -267,8 +266,7 @@ prepareSources verbosity tmp lbi isVersion2 bi args@HaddockArgs{argTargets=files
 
                  return hsFile
             needsCpp = EnableExtension CPP `elem` allExtensions bi
-            defines | isVersion2 = []
-                    | otherwise  = ["-D__HADDOCK__"]
+            defines = ["-D__HADDOCK__"]
 
 --------------------------------------------------------------------------------------------------
 -- constributions to HaddockArgs