Setting generateCacheFiles doesn't have any effect. Attempting to force-load the Qt.qmlcache module results in various 'variable not found' errors.
I'm not sure if auto-tests are supposed to catch this kind of regressions. This bug is easily reproducible using the cached-qml.qbs file from the qbs repository.
Git bisect shows that this bug was introduced in commit a5cc49f2c62cfd4094c6a8bccd7741ca9ee7e072 "Clean up the scope of configure scripts".