[MLton] Patch to build MLton on MinGW

[Matthew Fluet]

> Below is the patch I'm currently using to build MLton on MinGW.

Just to be clear -- this patch shouldn't be necessary to build MLton on 
MinGW.  Rather, it is a patch to get usable functionality out of MLNLFFI 
generated code that uses the MLNLFFI Library.

> The patch replaces the use of dl* functions to import malloc and
> free with MLton FFI code to import the functions.

That seems fine.

> The semantics of dlopen(NULL, /*...*/) seem difficult to emulate.

SML/NJ seems to have faked this issue by making the main lib NONE under a 
Unix system and making it SOME "kernel32.dll" under a win32 system.


> Index: runtime/platform/mingw.c
> ===================================================================
> --- runtime/platform/mingw.c	(revision 4860)
> +++ runtime/platform/mingw.c	(working copy)

> Index: lib/mlnlffi/memory/memalloc-a4-unix.sml
> ===================================================================
> --- lib/mlnlffi/memory/memalloc-a4-unix.sml	(revision 4860)
> +++ lib/mlnlffi/memory/memalloc-a4-unix.sml	(working copy)

Feel free to commit the patches.