We currently use a shell script to build the python externals, which means that we have no way to tell when things should be rebuilt. In addition, we build the externals directly to the dist dir, instead of the source dir, which means that none of the python code works from a source checkout. We should replace the shell script with a Makefile, and compile to lib/pegasus/externals/python instead of dist/pegasus-VERSION/lib/pegasus/externals/python.