Now we use Subversion, it seems excessive to have an individual $Id$

Now we use Subversion, it seems excessive to have an individual $Id$
line for every single .but file at the bottom of each page of the
HTML PuTTY docs. However, we can't _always_ replace that with a
single SVN revision, because there isn't always one available (SVN
still allows mixed working copies in which some files are
deliberately checked out against a different revision).

Hence, here's a mechanism for doing better. It uses `svnversion .'
to determine _whether_ a single revision number adequately describes
the current directory, and replaces all the version IDs with that if
so. If it can't do that, it uses the version IDs as before.

Also, this allows an explicit version string to be passed on the
make command line which will override _both_ these possibilities, so
that release documentation can be clearly labelled with the release
version number.

[originally from svn r4804]