fix portability bug: make sure destination file is writeable

Some shells let this go w/o comment, some don't.
Better to be sure.

(ensure): If "$dst" exists, make sure it is writeable.

contrib/gana-update.sh

@@ -22,6 +22,7 @@ ensure ()
 
     if ! diff $src $dst > /dev/null
     then
+        test ! -f $dst || chmod +w $dst
         cp $src $dst
         chmod -w $dst
     fi