finally really add back custom-setup stanza

Fourth or fifth try at this and finally found a way to make it work.

Absurd amount of busy-work forced on me by change in cabal's behavior.
Split up Utility modules that need posix stuff out of ones used by
Setup. Various other hacks around inability for Setup to use anything
that ifdefs a use of unix.

Probably lost a full day of my life to this.
This is how build systems make their users hate them. Just saying.

Utility/Path.hs

@@ -5,7 +5,7 @@
  - License: BSD-2-clause
  -}
 
-{-# LANGUAGE PackageImports, CPP #-}
+{-# LANGUAGE CPP #-}
 {-# OPTIONS_GHC -fno-warn-tabs #-}
 
 module Utility.Path where
@@ -17,13 +17,6 @@ import Data.Char
 import Control.Applicative
 import Prelude
 
-#ifdef mingw32_HOST_OS
-import qualified System.FilePath.Posix as Posix
-#else
-import System.Posix.Files
-import Utility.Exception
-#endif
-
 import Utility.Monad
 import Utility.UserInfo
 import Utility.Directory
@@ -247,50 +240,6 @@ dotfile file
   where
 	f = takeFileName file
 
-{- Converts a DOS style path to a msys2 style path. Only on Windows.
- - Any trailing '\' is preserved as a trailing '/' 
- - 
- - Taken from: http://sourceforge.net/p/msys2/wiki/MSYS2%20introduction/i
- -
- - The virtual filesystem contains:
- -  /c, /d, ...	mount points for Windows drives
- -}
-toMSYS2Path :: FilePath -> FilePath
-#ifndef mingw32_HOST_OS
-toMSYS2Path = id
-#else
-toMSYS2Path p
-	| null drive = recombine parts
-	| otherwise = recombine $ "/" : driveletter drive : parts
-  where
-	(drive, p') = splitDrive p
-	parts = splitDirectories p'
-	driveletter = map toLower . takeWhile (/= ':')
-	recombine = fixtrailing . Posix.joinPath
-	fixtrailing s
-		| hasTrailingPathSeparator p = Posix.addTrailingPathSeparator s
-		| otherwise = s
-#endif
-
-{- Maximum size to use for a file in a specified directory.
- -
- - Many systems have a 255 byte limit to the name of a file, 
- - so that's taken as the max if the system has a larger limit, or has no
- - limit.
- -}
-fileNameLengthLimit :: FilePath -> IO Int
-#ifdef mingw32_HOST_OS
-fileNameLengthLimit _ = return 255
-#else
-fileNameLengthLimit dir = do
-	-- getPathVar can fail due to statfs(2) overflow
-	l <- catchDefaultIO 0 $
-		fromIntegral <$> getPathVar dir FileNameLimit
-	if l <= 0
-		then return 255
-		else return $ minimum [l, 255]
-#endif
-
 {- Given a string that we'd like to use as the basis for FilePath, but that
  - was provided by a third party and is not to be trusted, returns the closest
  - sane FilePath.