Yet another build script update

* Changed Freetype retrieval from cvs to simple tarball download (much faster, no point in using static repo)
* SDL should also be changed to tarball download but 1.2.13 has appearently a bug in its "config-sdl" script (fails to list required libraries!). Let's wait for 1.2.14...
* OGLFT and BOINC are still fetched from their respective repos as there're no proper releases available anyway
* Also: added and implemented "--distclean" target