PDF: Use ; for TEXINPUTS separator on Windows.

Closes #1151, I hope.  Testing needed.

pandoc.cabal

@@ -243,6 +243,8 @@ Library
      cpp-options:   -DEMBED_DATA_FILES
      -- Build-Tools:   hsb2hs -- not yet recognized by cabal
      other-modules: Text.Pandoc.Data
+  if os(windows)
+    Cpp-options:      -D_WINDOWS
   Ghc-Options:   -rtsopts -Wall -fno-warn-unused-do-bind
   Ghc-Prof-Options: -auto-all -caf-all -rtsopts
   Default-Language: Haskell98

src/Text/Pandoc/PDF.hs

@@ -149,7 +149,12 @@ runTeXProgram program runsLeft tmpDir source = do
     let programArgs = ["-halt-on-error", "-interaction", "nonstopmode",
          "-output-directory", tmpDir, file]
     env' <- getEnvironment
-    let texinputs = maybe (tmpDir ++ ":") ((tmpDir ++ ":") ++)
+#ifdef _WINDOWS
+    let sep = ";"
+#else
+    let sep = ":"
+#endif
+    let texinputs = maybe (tmpDir ++ sep) ((tmpDir ++ sep) ++)
           $ lookup "TEXINPUTS" env'
     let env'' = ("TEXINPUTS", texinputs) :
                   [(k,v) | (k,v) <- env', k /= "TEXINPUTS"]