Cross-compilation targeting OSX

We surely could use the Travis CI also as our pipeline for packaging for darwin, but it would be better for OSX as a unix-based operating system to cross compile under linux and as a next step to also do the packaging on linux. This is something, that for sure we should also aim for when it comes to windows. But as we already figured out, this will take more effort and different approach to accomplish than for this one here for darwin.

We could also use something like a ready Cross compile toolchain. Or even something like a docker container working with osxcross for cross-compilation targeting darwin.