[m-rev.] for review: let host / target env types be overridden

[Julien Fischer]

On Tue, 30 Dec 2025 at 04:08, Julien Fischer <jfischer at opturion.com> wrote:
>
> Let host / target env types be overridden.
>
> Let the host and target env types be overridden in the environment.
> If not set in the environment, then they default to the values
> determined by the configure script.
>
> Use this capability to let Mercury compilers built in one of the MSYS2
> environments be usable from both the MSYS2 shell (via the mmc script) and
> also from cmd.exe (via the mercury.bat batch file), _without_ requiring any
> modifications to Mercury.config.

In the absence of any review comments, I intend to commit this later today.
I have other changes that are waiting on it.

Julien.