It should be possible to build UIMA-AS from the uima-as source alone, but unfortunately the uimaj 2.4.0 artifact it depends on was not uploaded. The work-around is to build uimaj first from the source, which is probably what you wanted anyway, but at this point in time the uima-as source in trunk depends on an earlier version of uimaj, so you must checkout and build the uimaj-2.4.0-rc1 tag of uimaj, not the trunk. That worked for me.
~Burn
