Ubuntu Fixes, 1.0 branch (2017.07.10.)

Various fixes to allow AnalysisBaseExternals to work out of the box on Ubuntu 16.04 and 17.04.

Note that projects may need to start using find_package(PythonInterp) explicitly after this update. Since find_package(PythonLibs) no longer sets up the PYTHONHOME environment variable for the built project. (As it can't do that reliably on Ubuntu, which has a different directory layout than LCG for instance.)