Added Text.Pandoc.Compat.Monoid.

This allows pandoc to compile with base < 4.5, where Data.Monoid
doesn't export `<>`.  Thanks to Dirk Ullirch for the patch.

pandoc.cabal

@@ -335,6 +335,7 @@ Library
                    Text.Pandoc.ImageSize,
                    Text.Pandoc.Slides,
                    Text.Pandoc.Highlighting,
+                   Text.Pandoc.Compat.Monoid,
                    Paths_pandoc
 
   Buildable:       True

src/Text/Pandoc/Compat/Monoid.hs

@@ -0,0 +1,16 @@
+{-# LANGUAGE CPP #-}
+module Text.Pandoc.Compat.Monoid ( Monoid(..)
+                                 , (<>)
+                          ) where
+
+#if MIN_VERSION_base(4,5,0)
+import Data.Monoid ((<>), Monoid(..))
+#else
+import Data.Monoid (mappend, Monoid(..))
+#endif
+
+#if MIN_VERSION_base(4,5,0)
+#else
+(<>) :: Monoid m => m -> m -> m
+(<>) = mappend
+#endif

src/Text/Pandoc/Templates.hs

@@ -102,7 +102,7 @@ import Control.Applicative
 import qualified Data.Text as T
 import Data.Text (Text)
 import Data.Text.Encoding (encodeUtf8)
-import Data.Monoid ((<>), Monoid(..))
+import Text.Pandoc.Compat.Monoid ((<>), Monoid(..))
 import Data.List (intersperse, nub)
 import System.FilePath ((</>), (<.>))
 import qualified Data.Map as M

src/Text/Pandoc/Writers/Docx.hs

@@ -35,7 +35,7 @@ import qualified Data.ByteString.Lazy as BL
 import qualified Data.ByteString.Lazy.Char8 as BL8
 import qualified Data.Map as M
 import qualified Text.Pandoc.UTF8 as UTF8
-import Data.Monoid ((<>))
+import Text.Pandoc.Compat.Monoid ((<>))
 import Codec.Archive.Zip
 import Data.Time.Clock.POSIX
 import Text.Pandoc.Definition