This is PR #4271, but with the changes to the descriptors, both the names of the
files and the names of the intermediate build artifact archives, removed.
This also closes#3775 if it goes in, because it covers the changes in
that PR.
The FTP server what we get libpng from only keeps the latest version in its main folder. Older versions are in the "history" folder. Apparently version 1.6.9 has been released, so 1.6.8 has moved to the history folder.
From a cursory glance, it looks like block 279,000 meets the criteria for
a checkpoint block, and it's also what the bootstrap.dat torrent is at. This closes#3530.
This commit adds a step, which is to git checkout the version to be
built. This ensures that the gitian-descriptors for said version will be
the correct ones. In addition, it updates the links for several
dependencies, where the previously existing links were dead.