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.

[originally from svn r9229]

mkunxarc.sh

@@ -1,4 +1,4 @@
-#!/bin/sh 
+#!/bin/bash 
 
 # Build a Unix source distribution from the PuTTY CVS area.
 #
@@ -16,7 +16,7 @@ case "$1" in
     ;;
   r*)
     arcsuffix="-$1"
-    ver="-DSVN_REV=$1"
+    ver="-DSVN_REV=${1#r}"
     docver=
     ;;
   '')