CMake configuration changes to build Gaudi with optional externals missing

In the last days I experimented a bit with teaching Gaudi how to build when only ROOT, Boost and TBB are available to it. This is the result. As far as I can tell the code builds in the same way as the current master branch when all possible externals are available. (All the ones available in a full ATLAS offline build at least.) And I'm also able to build the project in these modes:

• On Ubuntu 16.04 with the system provided Boost and TBB versions, against a privately built ROOT 6 version.
• On macOS Sierra against privately built Boost, ROOT and TBB versions. Both when building the project completely on its own, and when building it against an ATLAS externals project.

Some notes:

• The code uses C++14 expressions in quite a few places by now. So I raised the default C++ standard to C++14. This is the default in ATLAS builds since a while, I only saw this as an issue when building Gaudi "on its own" with a very minimal configuration command.
• While the code advertises that it still has support for ROOT 5, it's not able to build against it since a wile. Since the updates I put in for TLS variables on macOS. (The macros used there are ROOT 6 only.) And this update makes things even worse. As the code now relies on not providing the definition of some classes to the dictionary generator in GaudiPython that are not available. (Because AIDA or XercesC is missing.) While ROOT 6 handles this with just a build warning, ROOT 5's genreflex treats this with an ERROR message. That I was unable to get rid of.

So, extended testing is clearly necessary, the configuration code could definitely be fine tuned as I probably turned off the build of more source files than absolutely necessary, but I think we should move ahead with such a configuration organisation.