Only use cp -a if it is supported, falling back to cp -p or plain cp.

* cp --reflink=auto is used if supported, and will make git annex unlock
  much faster on filesystems like btrfs that support copy of write.

configure.hs

@@ -1,5 +1,4 @@
-{- Checks system configuration and generates SysConfig.hs.
- -}
+{- Checks system configuration and generates SysConfig.hs. -}
 
 import System.IO
 import System.Cmd
@@ -12,8 +11,9 @@ data Config = Config String Bool
 
 tests :: [TestDesc]
 tests = [
-	  TestDesc "cp -a" "cp_a" cp_a
-	, TestDesc "cp --reflink" "cp_reflink" cp_reflink
+	  TestDesc "cp -a" "cp_a" $ testCp "-a"
+	, TestDesc "cp -p" "cp_p" $ testCp "-p"
+	, TestDesc "cp --reflink=auto" "cp_reflink_auto" $ testCp "--reflink=auto"
 	]
 
 tmpDir :: String
@@ -25,11 +25,9 @@ testFile = tmpDir ++ "/testfile"
 quiet :: String -> String
 quiet s = s ++ " 2>/dev/null"
 
-cp_a :: Test
-cp_a = testCmd $ quiet $ "cp -a " ++ testFile ++ " " ++ testFile ++ ".new"
-
-cp_reflink :: Test
-cp_reflink = testCmd $ quiet $ "cp --reflink=auto " ++ testFile ++ " " ++ testFile ++ ".new"
+testCp :: String -> Test
+testCp option = testCmd $ quiet $ "cp " ++ option ++ " " ++ testFile ++ 
+			" " ++ testFile ++ ".new"
 
 testCmd :: String -> Test
 testCmd c = do
@@ -51,6 +49,7 @@ writeSysConfig config = do
 		header = [
 			  "{- Automatically generated by configure. -}"
 			, "module SysConfig where"
+			, ""
 			]
 		footer = []
 		vars [] = []
@@ -58,6 +57,7 @@ writeSysConfig config = do
 		showvar (Config name val) = [
 			  name ++ " :: Bool"
 			, name ++ " = " ++ show val
+			, ""
 			]
 
 runTests :: [TestDesc] -> IO [Config]