Don't recognize directories as libraries.  eg. If we have system
libvirt and OCaml bindings in $OCAML_LIB_DIR/libvirt, then we don't
want '-lvirt' to "discover" the OCaml bindings directory.

diff -ur flexdll/reloc.ml flexdll.mingw/reloc.ml
--- flexdll/reloc.ml	2008-11-16 18:41:01.000000000 +0000
+++ flexdll.mingw/reloc.ml	2008-11-16 18:39:18.000000000 +0000
@@ -109,7 +109,7 @@
   Filename.dirname (get_output1 (Printf.sprintf "i686-pc-mingw32-gcc %s-print-libgcc-file-name" extra))
 
 let file_exists fn =
-  if Sys.file_exists fn then Some fn
+  if Sys.file_exists fn && not (Sys.is_directory fn) then Some fn
   else if !use_cygpath && Sys.file_exists (fn ^ ".lnk") then
     Some (get_output1 (Printf.sprintf "cygpath -m %s" fn))
   else None