Build system changes

So... i changed some things in the build system that bothered me a bit:
- enabled Unicode in the projects (no explanation needed, right?)
- explicitly set the cmake policy for qt linking (this disables the masses of cmake warnings)
- enabled to pass in paths to 3rd-party dependencies via the -D switch of cmake or environment variables instead of always relying on environment variables

I stumbled across the 3rd 'problem' while setting up an automated build with jenkins... it was the easiest to just pass the necessary variables in the cmake call. It's now also easier to test with various versions of the libraries because you only have to adjust the path in the cmake call. Parallel builds with different library versions are therefore also possible.

Please check my changes and give me some feedback :)