Because there is no source version defined in the version.properties, the build process is at the mercy of JDK of the developer running the build. Currently the bobo project is compiled at API version 1.6 which means it can't be used on java 1.5.
There seems to be no reason for the limitation since it would appear that there are no 1.6 APIs used in the source. I rebuilt 2.5.1 from source with no issues.
The solution is simply to add: