Fix MSys2 side-conditions for conf-packages, part 2/3

[jmid]

This is the promised second PR out of 3, that solves MSys2 side conditions, as announced in #28939.

TLDR: since opam.2.2 MSys uses os = "win32" and os-distribution = "msys2" meaning that the MSys2 side-conditions for conf-packages are subtly wrong, see for example #28769 (review).

This second PR combs over and corrects the following conf-packages:
- conf-gtksourceview3
- conf-libevent
- conf-libffi
- conf-liblz4
- conf-libpcre2-8
- conf-libpcre
- conf-libssl
- conf-ncurses

[jmid]

CI spotted a couple of things, confirming that MSys2 CI was a good idea to add! 😄

First off, conf-libpcre was not installable!
https://github.com/ocaml/opam-repository/actions/runs/19573412563/job/56052318655?pr=28944

Testing conf-libpcre.2
  [ERROR] Package conflict!
    * Missing dependency:
  No solution found, exiting
      - conf-libpcre >= 2 -> conf-mingw-w64-pcre-i686 | conf-mingw-w64-pcre-x86_64
      unmet availability conditions: 'os = "win32" & os-distribution = "cygwin"'
      unmet availability conditions: 'os = "win32" & os-distribution = "cygwin"'
conf-libpcre.2 is not installable. Skip.

This turns out to be due to too strict availability guards, preventing anything but Cygwin-MinGW...

Secondly, conf-ncurses is failing:

Testing conf-ncurses.1
  The following actions will be performed:
  === install 2 packages
    - install conf-mingw-w64-ncurses-x86_64 1 [required by conf-ncurses]
    - install conf-ncurses                  1
  <><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
  [ERROR] The compilation of conf-mingw-w64-ncurses-x86_64.1 failed at "pkgconf --personality=x86_64-w64-mingw32 ncurses".
  #=== ERROR while compiling conf-mingw-w64-ncurses-x86_64.1 ====================#
  # context     2.4.1 | win32/x86_64 | ocaml.5.4.0 | file://D:/a/opam-repository/opam-repository
  # path        D:\opamroot\default\.opam-switch\build\conf-mingw-w64-ncurses-x86_64.1
  # command     C:\msys64\mingw64\bin\pkgconf.exe --personality=x86_64-w64-mingw32 ncurses
  # exit-code   1
  # env-file    D:\opamroot\log\conf-mingw-w64-ncurses-x86_64-9376-21713b.env
  # output-file D:\opamroot\log\conf-mingw-w64-ncurses-x86_64-9376-21713b.out
  <><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
  +- The following actions failed
  | - build conf-mingw-w64-ncurses-x86_64 1
  +- 
  - No changes have been performed
  # To update the current shell environment, run: (& opam env) -split '\r?\n' | ForEach-Object { Invoke-Expression $_ }
conf-ncurses.1 failed to build.

Looking at https://packages.msys2.org/packages/mingw-w64-x86_64-ncurses
I've discovered that the MSys2 package doesn't offer an ncurses.pc pkg-config file: 😮

/mingw64/lib/pkgconfig/formw.pc
/mingw64/lib/pkgconfig/menuw.pc
/mingw64/lib/pkgconfig/ncurses++w.pc
/mingw64/lib/pkgconfig/ncursesw.pc
/mingw64/lib/pkgconfig/panelw.pc

As such, this is actually working as intended, telling us there is no such file.
I'm uncertain how to fix this as I am no ncurses expert, and hence I don't know if "ncursesw" (what appear to be a wide variant) is good enough, if it is an MSys2 packaging failure, or if we should simply mark it as unavailable... 🤔
Polite ping @mtelvers as you added this originally. WDYT?

[jmid]

The MSys2 MinGW CI rerun is failing on

• conf-mingw-w64-gtksourceview3-i686 because the package is not available on i686 as also noted in the file
  # no i686 package available at https://packages.msys2.org/base/mingw-w64-gtksourceview3
• conf-libpcre2-8.1 is failing as the Windows support is added to conf-libpcre2-8.2.
  As such it is an expected failure.
  However I'm unsure why testing conf-mingw-w64-pcre-i686.1 causes
  downgrade conf-libpcre2-8 2 to 1 [upstream or system changes]
  and ultimately leading to this 🤔