CMakeLists: set variable to zero make it unavailable

[martink@xxxxxxxxx (Martin Kepplinger)]

hi Tuomo,

When I change the top-level CMakeLists.txt's version variable so that 
one is zero, like:
                    set(LIBTS_VERSION_REVISION 0)
I get:

[ 19%] Building C object src/CMakeFiles/tslib.dir/ts_version.c.o
/home/martin/dev/tslib/src/ts_version.c: In function ?ts_libversion?:
/home/martin/dev/tslib/src/ts_version.c:49:10: error: 
?LIBTS_VERSION_REVISION? undeclared (first use in this function)
   patch = LIBTS_VERSION_REVISION;


Could you look into this? Especially this variable must be possible to 
be 0.

thanks a lot,

                                           martin