I have printf-debugged. The problem is related to OS_NT being now a macro
in Msys2's toolchain. Thus, when Emacs is compiled, OS_NT = OS_9X = 1 !!!
See this patch that was submitted to my repo
https://github.com/juanjosegarciaripoll/emacs-build/blob/757284e60e13da8341e181b86cda4a7c0d5d079c/patches/0001-Using-other-enum-than-OS_NT-which-is-a-define-in-Win.patch

El dom., 16 may. 2021 21:14, Eli Zaretskii <eliz@gnu.org> escribió:

> > From: Juan José García-Ripoll
> >  <juanjose.garciaripoll@gmail.com>
> > Date: Sun, 16 May 2021 20:38:41 +0200
> >
> > Hi,
> >
> > I am trying to build emacs using my own script
> > (https://github.com/juanjosegarciaripoll/emacs-build) for cloning the
> > official repo, downloading MSYS2 and building from scratch.
> >
> > This which worked in the past but with a recent MSYS2 distribution,
> > Emacs does not seem to build. When it launches 'temacs' it aborts with
> > an error window that complains about UNICOWS.DLL not being available in
> > the system (which it is not).
>
> This means Emacs thinks you are on Windows 9X.  When did you last
> updated from Git, and which branch of the repository are you building?
>