Suppress missing haddock warning for some packages.

Allow a hardcoded whitelist of packages (currently 'rts') to not have
haddocks. When building several packages together, the accumulation of
these spurious warnings can make it a bit harder to notice legitimate
errors.

Cabal/Distribution/Simple/Haddock.hs

@@ -52,7 +52,9 @@ module Distribution.Simple.Haddock (
 
 -- local
 import Distribution.Package
-         ( PackageIdentifier, Package(..), packageName )
+         ( PackageIdentifier(..)
+         , Package(..)
+         , PackageName(..), packageName )
 import qualified Distribution.ModuleName as ModuleName
 import Distribution.PackageDescription as PD
          ( PackageDescription(..), BuildInfo(..), allExtensions
@@ -503,7 +505,9 @@ haddockPackageFlags lbi clbi htmlTemplate = do
           if exists
             then return (Right (interface, html))
             else return (Left (packageId ipkg))
-    | ipkg <- PackageIndex.allPackages transitiveDeps ]
+    | ipkg <- PackageIndex.allPackages transitiveDeps
+    , pkgName (packageId ipkg) `notElem` noHaddockWhitelist
+    ]
 
   let missing = [ pkgid | Left pkgid <- interfaces ]
       warning = "The documentation for the following packages are not "
@@ -515,6 +519,7 @@ haddockPackageFlags lbi clbi htmlTemplate = do
   return (flags, if null missing then Nothing else Just warning)
 
   where
+    noHaddockWhitelist = map PackageName [ "rts" ]
     interfaceAndHtmlPath :: InstalledPackageInfo -> Maybe (FilePath, FilePath)
     interfaceAndHtmlPath pkg = do
       interface <- listToMaybe (InstalledPackageInfo.haddockInterfaces pkg)