Deduplicate arguments that hsc2hs passes to gcc and ld

If extra-lib-dirs and/or extra-include-dirs are specified via either
command line or project files, they’re going to be added for each
package in the dendency graph. With enough long enough directories
the command-line length limits for gcc may be hit and calls to
the C compiler made from hsc2hs will start failing.

Ideally hsc2hs should be using repsonse files, but needlessly
specifying myriad of command-line parameters is redundant anyway.

Author: sergv

Cabal/src/Distribution/Simple/PreProcess.hs

@@ -520,7 +520,7 @@ ppHsc2hs bi lbi clbi =
         ++ postccldFlags
         ++ ["-o", outFile, inFile]
       where
-        ldflags = map ("--lflag=" ++) $ concat
+        ldflags = map ("--lflag=" ++) $ ordNub $ concat
           [ programDefaultArgs gccProg ++ programOverrideArgs gccProg
           , osxFrameworkDirs
           , [ arg
@@ -569,7 +569,7 @@ ppHsc2hs bi lbi clbi =
             ]
           ]
 
-        cflags = map ("--cflag=" ++) $ concat
+        cflags = map ("--cflag=" ++) $ ordNub $ concat
           [ programDefaultArgs gccProg ++ programOverrideArgs gccProg
           , osxFrameworkDirs
           , platformDefines lbi