Dear all,
swi-prolog is now included in the MSYS2 distribution, see screendump below. A few warnings show up during the compilation; I’ll post them after having had a look at swipl-devel (if they have already disappeared on swipl-devel, I consider the case as closed).
Best wishes,
Matthias “Professor MinGW” 
c7201178@PC105-C720 MINGW64 ~
$ pacman -Sl|grep prolog
mingw32 mingw-w64-i686-swi-prolog 8.4.2-1
mingw64 mingw-w64-x86_64-swi-prolog 8.4.2-1
ucrt64 mingw-w64-ucrt-x86_64-swi-prolog 8.4.2-1
c7201178@PC105-C720 MINGW64 ~
$ pacman -S mingw-w64-x86_64-swi-prolog
resolving dependencies...
looking for conflicting packages...
Packages (1) mingw-w64-x86_64-swi-prolog-8.4.2-1
Total Download Size: 3.31 MiB
Total Installed Size: 17.27 MiB
:: Proceed with installation? [Y/n]
:: Retrieving packages...
mingw-w64-x86_64... 3.3 MiB 4.67 MiB/s 00:01 [#####################] 100%
(1/1) checking keys in keyring [#####################] 100%
(1/1) checking package integrity [#####################] 100%
(1/1) loading package files [#####################] 100%
(1/1) checking for file conflicts [#####################] 100%
(1/1) checking available disk space [#####################] 100%
:: Processing package changes...
(1/1) installing mingw-w64-x86_64-swi-prolog [#####################] 100%
c7201178@PC105-C720 MINGW64 ~
$ swipl
Welcome to SWI-Prolog (threaded, 64 bits, version 8.4.2)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.
For online help and background, visit https://www.swi-prolog.org
For built-in help, use ?- help(Topic). or ?- apropos(Word).
member(1, X).
X = [1|_] ;
X = [_, 1|_] ;
X = [_, _, 1|_]
pack_install(environ).
% Contacting server at https://www.swi-prolog.org/pack/query ... ok
Install environ@1.0 from http://www.swi-prolog.org/download/pack/environ-1.0.tgz Y/n?
% Contacting server at https://www.swi-prolog.org/pack/query ... ok
% "environ-1.0.tgz" was downloaded 72 times
Package: environ
Title: Demo package with C code, fetching the program environment
Installed version: 1.0
Author: Jan Wielemaker <jan@swi-prolog.org>
Home page: http://www.swi-prolog.org/howto/ForeignPack.html
Download URL: http://www.swi-prolog.org/download/pack/environ-*.tgz
Install "environ-1.0.tgz" (1,084 bytes) Y/n?
% gcc -I"c:/msys64/mingw64/include" -c -o c/environ.o c/environ.c
% mkdir -p lib/x64-win64
% gcc -shared -L"c:/msys64/mingw64/bin" -o lib/x64-win64/environ.dll c/environ.o -lswipl
% make: Nothing to be done for 'check'.
% make: Nothing to be done for 'install'.
true.
use_module(library(environ)).
true.
halt.
c7201178@PC105-C720 MINGW64 ~