Rewrote free disk space checking code

Moving the portability handling into a small C library cleans up things
a lot, avoiding the pain of unpacking structs from inside haskell code.

configure.hs

@@ -1,23 +1,6 @@
 {- configure program -}
 
-import Data.Maybe
-
-import qualified Build.Configure as Configure
-import Build.TestConfig
-import Utility.StatFS
-
-tests :: [TestCase]
-tests = [ TestCase "StatFS" testStatFS
-	] ++ Configure.tests False
-
-{- This test cannot be included in Build.Configure due to needing
- - Utility/StatFS.hs to be built, which it is not when "cabal configure"
- - is run. -}
-testStatFS :: Test
-testStatFS = do
-	s <- getFileSystemStats "."
-	return $ Config "statfs_sanity_checked" $
-		MaybeBoolConfig $ Just $ isJust s
+import Build.Configure
 
 main :: IO ()
-main = Configure.run tests
+main = run tests