make dist: Make release tarball generation even more deterministic

With this commit the generated tarball doesn't depend on the user that
ran `make dist`.  It makes the tarballs identical whether run on my Debian
or my Ubuntu 12.04 docker container.

This is an extension to the work started in ce079fe.

Tested with:

    make -B dist && md5sum stb-tester-$(<VERSION).tar.gz