Don't nub lists of dependencies

It's pretty meaningless since it's only a syntactic check.
The proper thing is to maintain a dependency set or to
simplify dependencies before printing them.

Distribution/PackageDescription.hs

@@ -367,11 +367,11 @@ instance Monoid BuildInfo where
   }
   mappend a b = BuildInfo {
     buildable         = buildable a && buildable b,
-    buildTools        = combineNub buildTools,
+    buildTools        = combine    buildTools,
     cppOptions        = combine    cppOptions,
     ccOptions         = combine    ccOptions,
     ldOptions         = combine    ldOptions,
-    pkgconfigDepends  = combineNub pkgconfigDepends,
+    pkgconfigDepends  = combine    pkgconfigDepends,
     frameworks        = combineNub frameworks,
     cSources          = combineNub cSources,
     hsSourceDirs      = combineNub hsSourceDirs,

Distribution/PackageDescription/Configuration.hs

@@ -81,7 +81,6 @@ import qualified Distribution.Compat.ReadP as ReadP ( char )
 
 import Data.Char ( isAlphaNum )
 import Data.Maybe ( catMaybes, maybeToList )
-import Data.List  ( nub )
 import Data.Map ( Map, fromListWith, toList )
 import qualified Data.Map as M
 import Data.Monoid
@@ -451,10 +450,10 @@ finalizePackageDescription userflags mpkgs os arch impl constraints
       Right ((mlib, exes'), deps, flagVals) ->
         Right ( pkg { library = mlib
                     , executables = exes'
-                    , buildDepends = nub deps
+                    , buildDepends = deps
                     }
               , flagVals )
-      Left missing -> Left $ nub missing
+      Left missing -> Left missing
   where
     -- Combine lib and exes into one list of @CondTree@s with tagged data
     condTrees = maybeToList (fmap (mapTreeData Lib) mlib0 )
@@ -521,7 +520,7 @@ flattenPackageDescription :: GenericPackageDescription -> PackageDescription
 flattenPackageDescription (GenericPackageDescription pkg _ mlib0 exes0) =
     pkg { library = mlib
         , executables = reverse exes
-        , buildDepends = nub $ ldeps ++ reverse edeps
+        , buildDepends = ldeps ++ reverse edeps
         }
   where
     (mlib, ldeps) = case mlib0 of