[java] build avoidance -- again

[Steve Vinoski]

I noticed that the new code generation stuff again breaks build  
avoidance. It unconditionally generates code, which means the  
generated code has to be compiled every single time you build.

We've already fixed this problem twice. Every time the code  
generation stuff is modified, build avoidance is broken.

If you change something and run maven twice, the second time through,  
nothing should require compilation. If it does, you've broken  
something, and you need to fix it before committing.

--steve