add CPPFLAGS allowing zmq to resolve sodium

On a freshly installed Ubuntu 14.04 system the zeromq-buildbot fails to build libzmq properly because it fails to find sodium.h. After investigating this issue it appears that the zeromq-buildbot was succeeding on my test machine because it could fall back to system installed ZMQ components in some situations where settings were not defined properly.

This change sets the CPPFLAGS path to point to the buildbot include area which allows libzmq to find the correct sodium.h and build properly.