Added a wrinkle to the version.c build in the Unix makefile, which

Added a wrinkle to the version.c build in the Unix makefile, which
will validate an md5sums manifest and if all md5sums match will use
a version number provided in a file. This should allow me to produce
a Unix release source archive with the property that when unpacked
and built it will produce binaries advertising themselves as
`Release X.YZ', but as soon as the user starts fiddling with the
sources it will revert to `Unidentified build' (though of course the
user can still _explicitly_ ask for a release tag, and in fact this
will override the default if any default is specified).

git-svn-id: svn://svn.tartarus.org/sgt/putty@3818 cda61777-01e9-0310-a592-d414129be87e