This means we can add a trailing «\» to every element, so that they
can be removed without requiring modification of other lines, and can
be easily sorted.
Replace the old usage of $(nil) which could possibly end up with junk
added if such variable is ever defined, in the environment.
The ChangeLog file is distributed, and cannot be regenerated outside
of the git repository, so do not remove it in DISTCLEANFILES, and move
the generation code into dist-hook, which also avoids unnecessary
computation during normal builds.
