allow building w/o cryptohash

Mostly for the debian stable autobuilds, which have a too old version
to use the Crypto.Hash module.
(cherry picked from commit 20fb905bb6)

Utility/Hash.hs

@@ -1,13 +1,30 @@
 {- Convenience wrapper around cryptohash.
- -
- - The resulting Digests can be shown to get a canonical hash encoding. -}
+ - Falls back to SHA if it's not available.
+ -}
 
-module Utility.Hash where
+{-# LANGUAGE CPP #-}
+
+module Utility.Hash (
+	sha1,
+	sha224,
+	sha256,
+	sha384,
+	sha512,
+#ifdef WITH_CRYPTOHASH
+	skein256,
+	skein512,
+#endif
+	prop_hashes_stable
+) where
 
-import Crypto.Hash
 import qualified Data.ByteString.Lazy as L
 import qualified Data.ByteString.Char8 as C8
 
+#ifndef WITH_CRYPTOHASH
+import Data.Digest.Pure.SHA
+#else
+import Crypto.Hash
+
 sha1 :: L.ByteString -> Digest SHA1
 sha1 = hashlazy
 
@@ -33,6 +50,8 @@ skein256 = hashlazy
 skein512 :: L.ByteString -> Digest Skein512_512
 skein512 = hashlazy
 
+#endif
+
 {- Check that all the hashes continue to hash the same. -}
 prop_hashes_stable :: Bool
 prop_hashes_stable = all (\(hasher, result) -> hasher foo == result)
@@ -41,8 +60,10 @@ prop_hashes_stable = all (\(hasher, result) -> hasher foo == result)
 	, (show . sha256, "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae")
 	, (show . sha384, "98c11ffdfdd540676b1a137cb1a22b2a70350c9a44171d6b1180c6be5cbb2ee3f79d532c8a1dd9ef2e8e08e752a3babb")
 	, (show . sha512, "f7fbba6e0636f890e56fbbf3283e524c6fa3204ae298382d624741d0dc6638326e282c41be5e4254d8820772c5518a2c5a8c0c7f7eda19594a7eb539453e1ed7")
+#ifdef WITH_CRYPTOHASH
 	, (show . skein256, "a04efd9a0aeed6ede40fe5ce0d9361ae7b7d88b524aa19917b9315f1ecf00d33")
 	, (show . skein512, "fd8956898113510180aa4658e6c0ac85bd74fb47f4a4ba264a6b705d7a8e8526756e75aecda12cff4f1aca1a4c2830fbf57f458012a66b2b15a3dd7d251690a7")
+#endif
 	]
   where
 	foo = L.fromChunks [C8.pack "foo"]