[m-rev.] for review: rewrite the MinGW / MSYS / MSYS2 README

[Julien Fischer]

In the absence of any review comments I intend to commit this one
(and do the associated file renaming) later today.

Julien.

On Mon, 29 Dec 2025 at 20:52, Julien Fischer <jfischer at opturion.com> wrote:
>
> For review by anyone.
>
> I have attached the diff, but have also included the full file.  Reviewers
> should probably look at the latter since very little of what was previously
> present remains.
>
> I intend to rename README.MinGW to README.MSYS2.md before I
> push this change.  I will update links from other READMES at that point.
>
> -----------------------------------
>
> Rewrite the MinGW / MSYS / MSYS2 README.
>
> - Delete coverage of the old MSYS / MinGW platform, since it is dead
>   at this point. Recommend that users do not use it. This README now
>   describes the MSYS2 platform exclusively.
>
> - Delete a note to developers about the bootcheck script not working.
>   It has worked with MSYS2 since the beginning of 2025 and we no longer
>   care about the old MSYS.
>
> - Delete a section describing how to build a MinGW cross compiler
>   in Cygwin using gcc's -mno-cygwin option. That has not worked
>   in well over a decade. (In fact, I can't remember it ever having
>   worked.)
>
> - Describe MSYS2 environments and which ones we recommend for use
>   with Mercury.
>
> - Expand the description of how to use Mercury in the MSYS2 shell.
>
> - Add a section on a common problem, Mercury compiler generated
>   executables aborting due to the loader being unable to find
>   libwinpthread-1.dll.
>
> Documentation/README.MinGW:
>     As above.
>
> Julien.