diff -rN -u old-cabal/Distribution/Simple/GHC.hs new-cabal/Distribution/Simple/GHC.hs
--- old-cabal/Distribution/Simple/GHC.hs	2007-04-24 10:22:06.000000000 -0700
+++ new-cabal/Distribution/Simple/GHC.hs	2007-04-24 10:22:06.000000000 -0700
@@ -381,6 +381,9 @@
   let file = case makefileFile flags of
                 Just f ->  f
                 _otherwise -> "Makefile"
+  targetExists <- doesFileExist file
+  when targetExists $
+    die ("Won't overwrite existing copy of " ++ file)
   h <- openFile file WriteMode
 
   let Just lib = library pkg_descr