Ensure that packaging works on all target operating systems

[tschneidereit]

Ensure that packaging works on all target operating systems

[paulrouget]

Packaging is supposed to now work on the 3 platforms with ./mach package --release.

This required mostly changes in the python scripts. It's pretty dirty, but mach was not designed to be project agnostic. We will clean up all of that when we get rid of the python code: #33