Update roundTo to correspond to base-4.6.0.0 changes (gh-105)

Data/Text/Internal/Builder/RealFloat/Functions.hs

@@ -1,3 +1,5 @@
+{-# LANGUAGE CPP #-}
+
 -- |
 -- Module:    Data.Text.Internal.Builder.RealFloat.Functions
 -- Copyright: (c) The University of Glasgow 1994-2002
@@ -13,6 +15,30 @@ module Data.Text.Internal.Builder.RealFloat.Functions
     ) where
 
 roundTo :: Int -> [Int] -> (Int,[Int])
+
+#if MIN_VERSION_base(4,6,0)
+
+roundTo d is =
+  case f d True is of
+    x@(0,_) -> x
+    (1,xs)  -> (1, 1:xs)
+    _       -> error "roundTo: bad Value"
+ where
+  b2 = base `quot` 2
+
+  f n _ []     = (0, replicate n 0)
+  f 0 e (x:xs) | x == b2 && e && all (== 0) xs = (0, [])   -- Round to even when at exactly half the base
+               | otherwise = (if x >= b2 then 1 else 0, [])
+  f n _ (i:xs)
+     | i' == base = (1,0:ds)
+     | otherwise  = (0,i':ds)
+      where
+       (c,ds) = f (n-1) (even i) xs
+       i'     = c + i
+  base = 10
+
+#else
+
 roundTo d is =
   case f d is of
     x@(0,_) -> x
@@ -27,3 +53,5 @@ roundTo d is =
       where
        (c,ds) = f (n-1) xs
        i'     = c + i
+
+#endif