Fix version reporting in Unix builds versioned by a specific svn

Fix version reporting in Unix builds versioned by a specific svn
revision ('Custom build r1234'). Those builds were passing
'-DSVN_REV=r1234' to version.c, instead of -DSVN_REV=1234 as they
should have, leading to silly run-time version messages such as
'plink: Custom build rr9226'.

To chop the r off the front of the revision string passed in, I've
used a bashism in mkunxarc.sh. I think this is an acceptable extra
dependency.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9229 cda61777-01e9-0310-a592-d414129be87e