Clean up everything after successful build

[CodingMarkus]

As you cannot even run the script more than once (Build directory ... must not exist.), why would you want to keep the source or intermediate build files? If I want to re-run the script, I have to start all over anyway, so I have no advantage of keeping any of this; I cannot make incremental builds for example and if I'd know how to build all this stuff by hand, I wouldn't need your script.

All downloaded files should be deleted after download, that is the archives themselves as well as the unpacked source code after calling make install.

If I want the source code for some reason, I can just look at your script and see the download URL and download it manually.