The patch
[PATCH] Make FileSystemWatcher interface optional
It is still not fixed.
Recently, I didn't cross compile VICE, but that issue of autoconf is still open at Savannah, so I suspect that this is still valid.
This is a (incomplete) duplicate of patch #1295.
Thank you for the reply. Replacing all Q_OS_WINDOWS with Q_OS_WIN already fixed the issue on my build. I understand that Qt 5.9 branch is old, but I would like to say that nothing else needed to be changed besides using Q_OS_WIN instead of Q_OS_WINDOWS. Unfortunately, CYGWIN is shipped with Qt-5.9.4, which makes one of the above fixes the only solution. However, I can also provide a patch into my QSynth source package forCYGWIN, for making the build process possible.
Cannot compile anymore with QT5
All VICE emulators build fine again with SDL1. My executables for Windows 9x/Me/NT run again. Thank you very much.