Missing -I flag for windres on Windows
The build system needs to invoke windres with -I to specify the path where ghci.ico should be found. Omitting the flag appears to work with some windres versions but not others (since noone else has spotted this failure!).
Claus Reinke also had this issue (http://www.mail-archive.com/cvs-ghc\@haskell.org/msg14636.html) but it looks like he never had time to write a patch.
The attached patch fixes the Windows build for me (I'm using MingGW windres 2.17.50).