MacOS Updates, master branch (2019.04.16.)

Before looking into some possible updates of StatusCode, I wanted to revive a "standalone" Gaudi build on my laptop first. These are all the things that I had to do to make that happen.

The biggest technical update was in GaudiPluginService. Since on macOS neither <filesystem> nor <experimental/filesystem> is easily available, I decided to re-work the logic of the code a bit. So that it would use <filesystem> wherever it's available. Which, as far as I can see, is just GCC 8+ and Clang 7+.

https://en.cppreference.com/w/cpp/compiler_support

(MSVC also supports it with its most recent version, but we don't really care about MSVC, right?)

And for all other compilers the code rather picks up <boost/filesystem.hpp>. And just doesn't try to use <experimental/filesystem> under any circumstances. This seemed to be the simplest logic there...

I also had to make some tweaks around QMTest, as the python version that I use on my laptop doesn't support executing the script that the CMake configuration uses to detect the dependencies of the tests. So I updated the code to gracefully fail in this case.

With those technical difficulties solved, the rest of the updates were just to get rid of most of the compilation warnings on macOS (except for a few that I decided not to tackle), and to make the build with Xcode possible. The latter updates should help with other IDEs as well hopefully.