I'm using the MinGW version of OCaml 3.11.0 under Cygwin to build a
custom-linked executable.  The command

        ocamlc -custom -o foo.exe foo.ml

produces the rather cryptic error message

        Access is denied.
        ** Fatal error: hd
        File "_none_", line 1, characters 0-1:
        Error: Error while building custom runtime system
        
This problem has a simple solution: Cygwin allows multiple versions of
gcc to coexist by making /usr/bin/gcc.exe a symlink
to /etc/alternatives/gcc, and making another symlink from there
to /usr/bin/gcc-3.exe. The flexdll tool that's used to link OCaml
programs won't tolerate this.  So you need to remove the
symlink /usr/bin/gcc.exe and copy /usr/bin/gcc-3.exe in its place.  Then
everything works as it should.

Seasonal best wishes to all!

-- Mike Spivey