appveyor: fix build problem (dependency problem with ada/objc)

ada and objc are unavailable for msys2: for problem description see https://github.com/msys2/msys2/wiki/FAQ

.appveyor.yml

@@ -32,6 +32,8 @@ init:
 install:
   - ps: if (Test-Path Env:\CYG_ROOT) { Start-FileDownload "https://cygwin.com/$env:CYG_SETUP" -FileName "$env:CYG_SETUP" }
   - if defined CYG_ROOT (%CYG_SETUP% --quiet-mode --no-shortcuts --only-site --root "%CYG_ROOT%" --site "%CYG_MIRROR%" --local-package-dir "%CYG_CACHE%" --packages "%CYG_PACKAGES%" --upgrade-also)
+  # (temporary?) problem with msys/pacman/objc/ada (see https://github.com/msys2/msys2/wiki/FAQ)
+  - if defined MSYSTEM (%BASH% -lc "pacman -Rns --noconfirm mingw-w64-{i686,x86_64}-gcc-ada mingw-w64-{i686,x86_64}-gcc-objc")
   - if defined MSYSTEM (%BASH% -lc "pacman -Suuy --noconfirm")
   # the following line is not a duplicate line:
   # it is necessary to upgrade the MSYS base files and after that all the packages