Tests: Fix findPandoc to work with Windows.

pandoc.cabal

@@ -713,6 +713,8 @@ test-suite test-pandoc
                   Tests.Writers.FB2
                   Tests.Writers.Powerpoint
                   Tests.Writers.OOXML
+  if os(windows)
+    cpp-options:      -D_WINDOWS
   ghc-options:  -rtsopts -Wall -fno-warn-unused-do-bind -threaded
   default-language: Haskell2010
   other-extensions: NoImplicitPrelude

test/Tests/Helpers.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE FlexibleInstances    #-}
 {-# LANGUAGE TypeSynonymInstances #-}
@@ -100,7 +101,11 @@ findPandoc = do
              "test-pandoc" : "build" : ps
                -> joinPath (reverse ps) </> "build" </> "pandoc"
              _ -> error $ "findPandoc: could not find pandoc executable")
+#ifdef _WINDOWS
+        </> "pandoc" <.> "exe"
+#else
         </> "pandoc"
+#endif
   found <- doesFileExist pandocPath
   if found
      then return pandocPath