Fix search dirs used for pre-processing executables

It was combining the executable's hs-source-dirs with the library's
hs-source-dirs when pre-processing executables. This was inconsistent
with the search path used for finding ordinary .hs files and with
sdist and lead to confusing behaviour. See ticket #161.
If an executable needs source files from the library then it must specify
the right hs-source-dirs, including '.' if appropriate.
The behaviour is that if hs-source-dirs is not specified then the default
is '.' however if hs-source-dirs is specified then '.' is not implicitly
added. If it's needed then it must be added explicitly. This was always
the case for ordinary .hs files, this patch just brings things into line
for files that need pre-processing.

Distribution/Simple/PreProcess.hs

@@ -64,7 +64,7 @@ import Distribution.Simple.Compiler (CompilerFlavor(..), Compiler(..), compilerV
 import Distribution.Simple.LocalBuildInfo (LocalBuildInfo(..))
 import Distribution.Simple.Utils
          ( createDirectoryIfMissingVerbose, readUTF8File, writeUTF8File
-         , die, setupMessage
+         , die, setupMessage, intercalate
          , findFileWithExtension, findFileWithExtension', dotToSep )
 import Distribution.Simple.Program (Program(..), ConfiguredProgram(..),
                              lookupProgram, programPath,
@@ -77,7 +77,6 @@ import Distribution.Verbosity
 
 import Control.Monad (when, unless, join)
 import Data.Maybe (fromMaybe)
-import Data.List (nub)
 import System.Directory (getModificationTime)
 import System.Info (os, arch)
 import System.FilePath (splitExtension, dropExtensions, (</>), (<.>),
@@ -179,9 +178,7 @@ preprocessSources pkg_descr lbi forSDist verbosity handlers = do
         let bi = buildInfo theExe
         let biHandlers = localHandlers bi
         let exeDir = buildDir lbi </> exeName theExe </> exeName theExe ++ "-tmp"
-        sequence_ [ preprocessModule (nub $ (hsSourceDirs bi)
-                                  ++ (maybe [] (hsSourceDirs . libBuildInfo) (library pkg_descr)))
-                                     exeDir forSDist
+        sequence_ [ preprocessModule (hsSourceDirs bi) exeDir forSDist
                                      modu verbosity builtinSuffixes biHandlers
                   | modu <- otherModules bi]
         preprocessModule (hsSourceDirs bi) exeDir forSDist
@@ -213,7 +210,8 @@ preprocessModule searchLoc buildLoc forSDist modu verbosity builtinSuffixes hand
       Nothing -> do
                  bsrcFiles <- findFileWithExtension builtinSuffixes searchLoc (dotToSep modu)
                  case bsrcFiles of
-	          Nothing -> die ("can't find source for " ++ modu ++ " in " ++ show searchLoc)
+	          Nothing -> die ("can't find source for " ++ modu ++ " in "
+		                   ++ intercalate ", " searchLoc)
 	          _       -> return ()
         -- found a pre-processable file in one of the source dirs
       Just (psrcLoc, psrcRelFile) -> do