Did you follow the guide that Adam mentioned earlier?

https://www.labkey.org/home/Documentation/wiki-page.view?name=build

Although you're using the actual SRC package rather than using SVN to pull the source code from hedgehog.fhcrc.org, the work in the end is the same. The core.jar would get automatically created upon compiling and automatically create that core.jar into the ./build/deploy/modules/core/lib/ directory.

Regards,

Jon