Build the release tarball from git, not manually

Build the release tarball from git, not manually

Previously the Makefiles contained lists of files to distribute.
They built the release tarball manually. It's much easier to just
use git, which knows which files are the relevant files, because
it's the ones that are under version control. We just need to
add some additional, pre-generated files for convenience, so that
the user does not need to have auto* and lex installed.

The release tarball is now fully assembled in the top-level
Makefile. (The only tricky part is sbr/dtimep.c, because that
needs to be generated in the subdirectory.)