Fix Windows build

• /etc/gtk-4.0 no longer exists, make the install step ignore this
• add missing dependency enchant that is expected by the install step
• Remove atk from install step, which is no longer needed

Fixes inbox#10167 (closed)

• Depends on a newer gtkmm package being published: https://github.com/msys2/MINGW-packages/pull/20514 . In the meantime you can build that yourself: clone the above, cd gtkmm-4.0; makepkg
• Tested in a not 100% clean environment