QBS rebuilds all moc files if a qbs file of a subproduct or an environment variable changed.

While trying to compile qt-creator with qbs (to get qbs support btw). I had to edit some qbs files to make it working (they were outdated apparently) and set some environment variables (QBS_SOURCE_DIR, QBS_BUILD_DIR). Each time I changed either a qbs file (even a small module only) or an environment variable qbs rebuilt all moc files taking ages each time.