Rework ant version properties

Building on @rleigh-dundee's recent changes, this
refactors the work from build.py into a python
script like source-archive. That with the aid of
```
   <var name="x" unset="true"/>
```
elements, it's possible to remove the requirement
to use build.py.

----
--exclude
Marking this `exclude` to give @rleigh-dundee time to take a look. This is in preparation for further maven improvements on the `dev_5_0` branch. For example, note that the `parse_version` script uses `SNAPSHOT` and jumps to the next version.