Fix automatic version numbering in the Unix tarball.

Manfred Schwarb points out that when I moved the autoconf machinery up
from the unix subdirectory to the top level, in r10141, I missed a
couple of lingering $(srcdir)/.. in the make rule for version.o, as a
result of which the automatic checking of the manifest wasn't doing
its thing and tools built from a standard .tar.gz were reporting as
'Unidentified build'.


git-svn-id: svn://svn.tartarus.org/sgt/putty@10201 cda61777-01e9-0310-a592-d414129be87e