cgal is always re-built after being cmaked

When running cmake on cgal again after building it, cgal is built again instead of skipping the already built targets. This also caused simox-cgal to be built on each update.

At the moment, this is not too much of a problem as the packages are small and quickly built (on my machine at least). But maybe we want to consider fixing cgal or allowing to mark and handle such packages appropriately.