Use printf rather than echo to generate \versionid.

(Sacrificing the ability to build PuTTY docs from Halibut source on SunOS 4,
apparently, but I expect we'll live.)


git-svn-id: svn://svn.tartarus.org/sgt/putty@10102 cda61777-01e9-0310-a592-d414129be87e