add popCount for old version

Crypto/KDF/Scrypt.hs

@@ -9,6 +9,7 @@
 --
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE ForeignFunctionInterface #-}
+{-# LANGUAGE CPP #-}
 module Crypto.KDF.Scrypt
     ( Parameters(..)
     , generate
@@ -43,6 +44,12 @@ data Parameters = Parameters
 foreign import ccall "cryptonite_scrypt_smix"
     ccryptonite_scrypt_smix :: Ptr Word8 -> Word32 -> Word64 -> Ptr Word8 -> Ptr Word8 -> IO ()
 
+#if !(MIN_VERSION_base(4,5,0))
+popCount n = loop 0 n
+  where loop c 0 = c
+        loop c i = loop (c + if testBit c 0 then 1 else 0) (i `shiftR` 1)
+#endif
+
 -- | Generate the scrypt key derivation data
 generate :: Parameters -> B.ByteString
 generate params