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/Process/Transcript.hs

@@ -0,0 +1,87 @@
+{- Process transcript
+ -
+ - Copyright 2012-2015 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+{-# LANGUAGE CPP #-}
+{-# OPTIONS_GHC -fno-warn-tabs #-}
+
+module Utility.Process.Transcript where
+
+import Utility.Process
+
+import System.IO
+import Control.Concurrent
+import qualified Control.Exception as E
+import Control.Monad
+#ifndef mingw32_HOST_OS
+import qualified System.Posix.IO
+#else
+import Control.Applicative
+#endif
+import Data.Maybe
+import Prelude
+
+-- | Runs a process, optionally feeding it some input, and
+-- returns a transcript combining its stdout and stderr, and
+-- whether it succeeded or failed.
+processTranscript :: String -> [String] -> (Maybe String) -> IO (String, Bool)
+processTranscript cmd opts = processTranscript' (proc cmd opts)
+
+processTranscript' :: CreateProcess -> Maybe String -> IO (String, Bool)
+processTranscript' cp input = do
+#ifndef mingw32_HOST_OS
+{- This implementation interleves stdout and stderr in exactly the order
+ - the process writes them. -}
+	(readf, writef) <- System.Posix.IO.createPipe
+	readh <- System.Posix.IO.fdToHandle readf
+	writeh <- System.Posix.IO.fdToHandle writef
+	p@(_, _, _, pid) <- createProcess $ cp
+		{ std_in = if isJust input then CreatePipe else Inherit
+		, std_out = UseHandle writeh
+		, std_err = UseHandle writeh
+		}
+	hClose writeh
+
+	get <- mkreader readh
+	writeinput input p
+	transcript <- get
+
+	ok <- checkSuccessProcess pid
+	return (transcript, ok)
+#else
+{- This implementation for Windows puts stderr after stdout. -}
+	p@(_, _, _, pid) <- createProcess $ cp
+		{ std_in = if isJust input then CreatePipe else Inherit
+		, std_out = CreatePipe
+		, std_err = CreatePipe
+		}
+
+	getout <- mkreader (stdoutHandle p)
+	geterr <- mkreader (stderrHandle p)
+	writeinput input p
+	transcript <- (++) <$> getout <*> geterr
+
+	ok <- checkSuccessProcess pid
+	return (transcript, ok)
+#endif
+  where
+	mkreader h = do
+		s <- hGetContents h
+		v <- newEmptyMVar
+		void $ forkIO $ do
+			void $ E.evaluate (length s)
+			putMVar v ()
+		return $ do
+			takeMVar v
+			return s
+
+	writeinput (Just s) p = do
+		let inh = stdinHandle p
+		unless (null s) $ do
+			hPutStr inh s
+			hFlush inh
+		hClose inh
+	writeinput Nothing _ = return ()