defining make vars

[Graham Percival]

I'm looking at automatically replacing download links in
general.texi (and subfiles) with macros.  I think that's the best
way to deal with them; we already have the @version macro that's
auto-generated.

@version is defined from TOPLEVEL_VERSION, which in turn is
defined from TOPLEVEL_MAJOR_VERSION (etc).  But how is
TOPLEVEL_MAJOR_VERSION defined?  I'm guessing that one of the
makefiles or stepmake stuff actually reads from /VERSION, but I
can't figure out which piece of the build system does it.


I'm tempted to just dump the raw numbers into
  stepmake/stepmake/texinfo-rules.make
for now -- at least the texinfo files can be generic (i.e. using
proper macros, which would be defined in version.itexi).  And I
don't mind updating that file manually for a month or so.

Cheers,
- Graham