CMake 3.13 Updates, master branch (2019.01.14.)

@fwinkl, since you did some work in this area in the past, let me cc you on this.

It's been bugging me since a while that ever since CMake 3.13 came out, I've been seeing warnings about CMP0058. Telling me that the OLD behaviour of that policy will be removed from CMake soon.

So I took the opportunity to start some long-standing updates in the code.

• Turns out that "fixing" CMP0058 was actually pretty simple. I just had to introduce a dummy add_custom_command call into the code that convinces the Ninja generator that the stamp files that it has been complaining about in the past, will be in place, it shouldn't worry about them.
• For CMP0074 however I had to do a lot more. I now changed the naming convention, so that from now on AtlasLCG creates cache variables called <Foo>_LCGROOT and <Foo>_LCGVERSION to set up where LCG packages should be picked up from. And then I tried to update all parts of the code to start using the new convention.

Unfortunately this convention about <Foo>_ROOT variables is embedded very deeply into our CMake code. I'm pretty sure that even atlas/athena hardcodes this convention in a few places. So I still have to do a lot of tests before pushing this in even just into this repository. Still, I wanted to open the MR already now, thinking that you may have some feedback about all of this Frank.