Our compiler will think that os_type = "Win32".  Unfortunately in
the default OCaml this has the negative effect of causing it to
use '\' character in paths.  Since it's really running on a Linux
kernel, that won't work.  This is a quick and dirty fix.

Index: build-tree/ocaml-3.12.1/stdlib/filename.ml
===================================================================
--- build-tree.orig/ocaml-3.12.1/stdlib/filename.ml	2010-01-20 17:26:46.000000000 +0100
+++ build-tree/ocaml-3.12.1/stdlib/filename.ml	2011-10-09 04:22:47.684503451 +0200
@@ -68,7 +68,7 @@
 module Win32 = struct
   let current_dir_name = "."
   let parent_dir_name = ".."
-  let dir_sep = "\\"
+  let dir_sep = "/"
   let is_dir_sep s i = let c = s.[i] in c = '/' || c = '\\' || c = ':'
   let rindex_dir_sep s =
     let rec pos i =
