Only clean manpages with maintainer-clean.

We don't distribute the source files for rebuilding these, so it makes
no sense to clean them, or even distclean them. They're not intended
to be rebuilt be users of the tarball, only by people working from
the git sources.
Move the ChangeLog generation out of the Makefile.

Having it in the Makefile broke the distcheck target. The specific
failure case is if you do a VPATH configure, and then use the make
dist target, it regenerates the ChangeLog file rather than using the
one in $(srcdir) as it does for other distributed files. The reason it
does this is because there's a ChangeLog target in the Makefile.

This doesn't feel like a perfect solution, but it only makes life a
little harder for devs, and fixes potential breakages for users.
For consistent behaviour add config.{guess,sub}.

This makes it consistent across build platforms, and ensures that
anyone checking out our Git repository gets the same versions of these
files as we do. They match upstream so can be updated when needed.

It'd be nice to be able to rely on the operating system provided ones
being current, but at least in FreeBSD they're replaced by old ones. I
consider that a bug, but it's not been resolved yet.

They're sourced from here:

http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD
http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD