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/Rsync.hs

@@ -13,6 +13,10 @@ import Common
 import Utility.Metered
 import Utility.Tuple
 
+#ifdef mingw32_HOST_OS
+import qualified System.FilePath.Posix as Posix
+#endif
+
 import Data.Char
 import System.Console.GetOpt
 
@@ -139,3 +143,29 @@ filterRsyncSafeOptions = fst3 . getOpt Permute
 	[ Option [] ["bwlimit"] (reqArgLong "bwlimit") "" ]
   where
 	reqArgLong x = ReqArg (\v -> "--" ++ x ++ "=" ++ v) ""
+
+{- 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
+