How did you try that? Is there maybe a different /usr/include/audacious2 where the files for 2.x are installed? If that is the case, you may have to change line 18 and 39 in configure.in to say audacious2 instead of audacious and rerun bootstrap. There may be other changes necessary but that would be a start.
I added a Repo to get audacious 3. And after trying and breaking my auadacious so badly it won't start anymore :( I installed Xubuntu (allways wanted to try it) on my Desktop-PC. In the Ubuntu Repo is still audacious 2.44.
Unfortunaly there is no audacious2 folder in /usr/include. There are a bunch of header files in the audacious folder but no util.h. Searching for the file brings no satisfying results.
Ok, apparently util.h was removed in 2.4. Unfortunately just removing that include doesn't work, it is at least needed for the InputPlayback struct. I've got 2.3. I'll take a look and see if I can fix things for 2.4.