split PREFIX into PREFIX and DESTDIR

using a `PREFIX` that is relative to checkout tree is unwise as `PYTHONPATH` and `PYTHONHOME` will point there after compilation
all relative stuff was moved to `DESTDIR` and `PREFIX=/` now, because this where we mount our `python.tar` when executing it
Merge pull request #7 from pkit/minor_fixes

split PREFIX into PREFIX and DESTDIR