this is the custom-setup I want

Unfortunately, cabal fails to parse this.
https://github.com/haskell/cabal/issues/4852

git-annex.cabal

@@ -306,6 +306,8 @@ source-repository head
 custom-setup
   Setup-Depends: base (>= 4.5), hslogger, split, unix-compat, process,
     filepath, exceptions, bytestring, directory, IfElse, data-default, Cabal
+  if os(Windows)
+    Setup-Depends: Win32
 
 Executable git-annex
   Main-Is: git-annex.hs