When invoking hsc2hs, mention 'cabal_macros.h' only once.

Before & after: https://gist.github.com/23Skidoo/7220763

Cabal/Distribution/Simple/PreProcess.hs

@@ -453,6 +453,9 @@ ppHsc2hs bi lbi =
        ++ [ "--cflag=-I" ++ dir | dir <- PD.includeDirs  bi ]
        ++ [ "--cflag="   ++ opt | opt <- PD.ccOptions    bi
                                       ++ PD.cppOptions   bi ]
+       ++ [ "--cflag="   ++ opt | opt <-
+               [ "-I" ++ autogenModulesDir lbi,
+                 "-include", autogenModulesDir lbi </> cppHeaderName ] ]
        ++ [ "--lflag=-L" ++ opt | opt <- PD.extraLibDirs bi ]
        ++ [ "--lflag=-Wl,-R," ++ opt | isELF
                                 , opt <- PD.extraLibDirs bi ]
@@ -463,9 +466,7 @@ ppHsc2hs bi lbi =
        ++ [ "--cflag=" ++ opt
           | pkg <- pkgs
           , opt <- [ "-I" ++ opt | opt <- Installed.includeDirs pkg ]
-                ++ [         opt | opt <- Installed.ccOptions   pkg ]
-                ++ [ "-I" ++ autogenModulesDir lbi,
-                     "-include", autogenModulesDir lbi </> cppHeaderName ] ]
+                ++ [         opt | opt <- Installed.ccOptions   pkg ] ]
        ++ [ "--lflag=" ++ opt
           | pkg <- pkgs
           , opt <- [ "-L" ++ opt | opt <- Installed.libraryDirs    pkg ]