Better warning message when -fterminfo doesn't work.

Setup.hs

@@ -106,6 +106,11 @@ iconv_prog = unlines $
     , "}"
     ]
     
-warnIfNotTerminfo flags = when (not (hasFlagSet flags (FlagName "terminfo"))) $
-  putStrLn $
-    "*** Warning: running on POSIX but not building the terminfo backend. ***"
+warnIfNotTerminfo flags = when (not (hasFlagSet flags (FlagName "terminfo")))
+    $ mapM_ putStrLn
+    [ "*** Warning: running on POSIX but not building the terminfo backend. ***"
+    , "You may need to install the terminfo package manually, e.g. with"
+    , "\"cabal install terminfo\"; or, use \"-fterminfo\" when configuring or"
+    , "installing this package."
+    ,""
+    ]