qt: use qmlcachegen by default

If qmlcachegen is available, it should be used by default. Most distributions are going to omit this due to negligence if it is left as an opt-in option, e.g. https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=vlc-git.

Are we sure that Debian unstable CI uses qmlcachegen at the moment? qmlcachegen is useful for checking errors in QML files. Since I made it used by default here, I did not check the scripts if they use --enable-qt-qml-cache.

This was originally proposed here: !5737 (comment 446286).

Request review @chub.