Currently the Ubuntu slaves are missing one or more build dependencies for the C++ broker.
According to http://apache.org/dev/machines.html, the Ubuntu slaves run Ubuntu 10.04.x. By running bootstrap/configure on clean installs of 10.04.3, I can see that the minimum set of additional packages that need to installed are:
sudo apt-get install subversion automake autoconf libtool ruby sasl2-bin uuid-dev g++ libicu42 libstdc++6-4.4-dev python-dev python2.6-dev libboost1.40-dev libboost1.40-dev libboost-program-options-dev libboost-filesystem-dev
By comparing this list with the list of packages installed on Ubuntu1 (obtained by running 'dpkg' from Jenkins), I can see that libboost-filesystem-dev, sasl2-bin and uuid-dev are three missing dependencies.
I'll put in a request with firstname.lastname@example.org and see if these packages can be installed.