XRootD GCC 8 Fix, master branch (2019.06.12.)

This is to fix a compilation issue with the "standalone" build of XRootD with GCC 8 that I've known about for a while now. After some trial-and-error I decided that the best way to avoid the warning (which turns into an error because of -Werror) that shows up is to use a GCC #pragma to disable that warning in just the one file that is affected. (I didn't want to go further, and just disable it for the line affected. Disabling it for the entire file seemed good enough.)

At the same point I updated the AnalysisBaseExternals configuration to force the build of Python on CentOS 7 by default. I was debating for a while whether the CI system should do this instead, but that would've meant adding a project specific build flag in the CI system just for this one project. It seemed better to just make this tweak in the CMake configuration. Since we'll want to build Python by default in nightly builds as well, once we get to that stage...

This is to fix the CI build errors observed currently. For instance in !485 (merged).