Make cabal require hsb2hs >= 0.3.1.

This is done by adding `hookedPrograms` in `Setup.hs`,
which allows us to include `hsb2hs` in Build-Tools in cabal.

INSTALL

@@ -119,7 +119,7 @@ assume that the pandoc source directory is your working directory.
     - `embed_data_files`: embed all data files into the binary (default no).
       This is helpful if you want to create a relocatable binary.
       Note:  if this option is selected, you need to install the
-      `hsb2hs` preprocessor: `cabal install hsb2hs` (version 0.3 or
+      `hsb2hs` preprocessor: `cabal install hsb2hs` (version 0.3.1 or
       higher is required).
 
     - `https`:  enable support for downloading resources over https

Setup.hs

@@ -23,18 +23,35 @@ import Distribution.PackageDescription (PackageDescription(..), FlagName(..))
 import System.Process ( rawSystem )
 import System.FilePath ( (</>) )
 import System.Directory ( findExecutable )
+import Distribution.Verbosity ( Verbosity )
 import Distribution.Simple.Utils (info, notice, installOrdinaryFiles)
 import Distribution.Simple.Setup
+import Distribution.Simple.Program (simpleProgram, Program(..))
 import Distribution.Simple.LocalBuildInfo
+import Data.Version
+import System.Process (readProcess)
+import Text.ParserCombinators.ReadP (readP_to_S, skipSpaces, eof)
 import Control.Monad (when)
 
 main :: IO ()
 main = defaultMainWithHooks $ simpleUserHooks {
       -- enable hsb2hs preprocessor for .hsb files
       hookedPreProcessors = [ppBlobSuffixHandler]
+    , hookedPrograms = [(simpleProgram "hsb2hs"){
+                           programFindVersion = findHsb2hsVersion }]
     , postCopy = installManPage
     }
 
+findHsb2hsVersion :: Verbosity -> FilePath -> IO (Maybe Version)
+findHsb2hsVersion verb fp = do
+  outp <- readProcess fp ["--version"] ""
+  case readP_to_S (do v <- parseVersion
+                      skipSpaces
+                      eof
+                      return v) outp of
+       ((v,""):_) -> return (Just v)
+       _          -> return Nothing
+
 ppBlobSuffixHandler :: PPSuffixHandler
 ppBlobSuffixHandler = ("hsb", \_ lbi ->
   PreProcessor {

pandoc.cabal

@@ -292,7 +292,7 @@ Library
      cpp-options:   -DHTTP_CLIENT
   if flag(embed_data_files)
      cpp-options:   -DEMBED_DATA_FILES
-     -- Build-Tools:   hsb2hs -- not yet recognized by cabal
+     Build-Tools:   hsb2hs >= 0.3.1
      other-modules: Text.Pandoc.Data
   if os(windows)
     Cpp-options:      -D_WINDOWS