[m-rev.] for review: Update README.MinGW regarding MSYS2 and x86_64.

[Peter Wang]

On Fri, 28 Aug 2015 15:25:16 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> 
> Hi Peter,
> 
> On Tue, 25 Aug 2015, Peter Wang wrote:
> 
> > If anyone would care to try to reproduce the build on i686 or x86_64,
> > that would be great.
> 
> The only additional issue I encountered is that on my system there are
> three MSYS2 items under the start menu:
> 
>      * MinGW-w64 Win32 Shell
>      * MinGW-w64 Win64 Shell
>      * MSYS2

I'll clarify that.

> If you select the last one then GCC doesn't appear to be in the PATH
> by default.  I also had to apply the following patch:
> 
> --------------------
> 
> diff --git a/boehm_gc/Makefile.direct b/boehm_gc/Makefile.direct
> index 23c12bb..58e0cd7 100644
> --- a/boehm_gc/Makefile.direct
> +++ b/boehm_gc/Makefile.direct
> @@ -235,7 +235,7 @@ all: gc.a gctest
>   # Mercury-specific: clear DESTDIR
>   $(AO_INSTALL_DIR):
>          CC="$(CC)" AR="$(AR)" RANLIB="$(RANLIB)" MAKE=$(MAKE) DESTDIR= \
> -          $(srcdir)/build_atomic_ops.sh --host=$(FULLARCH)
> +          $(srcdir)/build_atomic_ops.sh --build=$(FULLARCH) --host=$(FULLARCH)
> 
>   LEAKFLAGS=$(CFLAGS) -DFIND_LEAK
> 
> --------------
> 
> I'm not quite sure *why* I had I to do that, although I suspect the
> reason is that the versions of config.guess and config.sub under the
> boehm_gc directory are *very* old (from ~2009).

I might have updated them in my workspace without knowing.
We can update the files in the boehm_gc and libatomic_ops directories
to the same versions we have in the root directory.

Thanks.

Peter