build: Enable/disable WITH_XDG_DIRS_FALLBACK based on Qt version (#111)
build: Enable/disable WITH_XDG_DIRS_FALLBACK based on Qt version (#111)

Use homemade Xdg dirs settings fallback by default if Qt < 5.9.0 and
force disabling the workadound for Qt >= 5.9.0 (as from 5.9.0 the
proper XDG_CONFIG_DIRS support is directly in QSettings).
lxde/liblxqt,lxde/liblxqt