Conditionally import Data.Monoid.<> depending on version of base

Fixes #64

app/Main.hs

@@ -1,6 +1,7 @@
 module Main where
 
 import           App (runProgram, Options(Options))
+import           Common
 import qualified Data.Maybe as M
 import           Options.Applicative
 import           Unused.CLI (SearchRunner(..))

src/Common.hs

@@ -0,0 +1,9 @@
+{-# LANGUAGE CPP #-}
+
+module Common
+    ( (<>)
+    ) where
+
+#if MIN_VERSION_base(4, 8, 0)
+import Data.Monoid ((<>))
+#endif

unused.cabal

@@ -59,6 +59,7 @@ library
                      , Unused.CLI.ProgressIndicator
                      , Unused.CLI.ProgressIndicator.Internal
                      , Unused.CLI.ProgressIndicator.Types
+                     , Common
   build-depends:       base >= 4.7 && < 5
                      , process
                      , containers