The build was failing on some machines because of a `find` command that
touched files in different orders on different machines.
That confused `make`s timestamp mechanism.
This "un-breaks" sage while also updating it to 8.0.
It compiles sage with its dependencies as one big pile, which is not
the best approach but definately better than nothing for now.
To be able to shrink the huge output pile a little, it also splits
docs from the rest of the output.