Incremental compilation and installation w/o root

[stefanct]

Hi,
the top-level makefile does not only compile but also (tries to) install everything. Moreover, it does start all over again because it removes all previous compilation results. This is not the typical Unix workflow at all and very annoying. It would be better if (at least optionally) the install step could be triggered individually and previous compilation results are not cleaned unconditionally on each run.