Davix Build Update, 1.0 branch (2018.10.25.)

All the hoopla around https://sft.its.cern.ch/jira/browse/ROOT-9739 was at least good for one thing. It made me realise that our build configuration for Davix needs to be smarter.

Until now the build of Davix was only attempted on SLC6 and CC7, making the decision of whether to attempt the build, purely based on the platform name. But it turns out that ROOT is prone to building Davix itself if it deems it possible. So it's a better idea for us to do the same, and provide our own version of Davix to ROOT whenever it can be provided.

So this update now explicitly looks for all requirements of the Davix build, and enables it if everything is in place. On macOS I still explicitly disable it, as I don't want to add our own build of OpenSSL to AnalysisBaseExternals... (So on macOS I'll just continue allowing ROOT to do whatever it deems best. Since ROOT does internally build OpenSSL as well when necessary.)

Pinging @elmsheus as one of the people who was behind our custom build of Davix in the first place.