macOS Catalina Updates, 1.0 branch (2019.10.13.)

As one could imagine, AnalysisBaseExternals does not build out of the box on Catalina. In this MR I'll try to make all the necessary fixes/updates.

First off, I had to make Python compile. Headers under /usr/include can no longer be made available on the new OS. So finally the Python build had to be updated to make use of the SDK installed under ${CMAKE_OSX_SYSROOT}. While trying to keep the build on all other platforms as it was.

I tried, both the PyModules package, and ROOT managed to build afterwards. Unfortunately I couldn't test the entire build because of issues with http://cern.ch/atlas-software-dist-eos/externals/ at the time of writing. 😦 And in any case, a lot more testing will have to be done with the changes on all other "production platforms" (in which I'm also including Ubuntu 18.04...) as well.

Cc. @krumnack, just to keep you informed about the progress.