Try building with the version of ant checked into the project. E.g.

set ANT_HOME=C:\labkey23\external\ant
set PATH=%PATH%;%ANT_HOME%\bin

There are a lot of strange warnings in this log, culminating in

    java.lang.NoClassDefFoundError: com/sun/mirror/apt/AnnotationProcessorFactory

Josh may have some more insight as to why this happens, but I think this is essentially an ant configuration problem.