The osxcross installation process has been improved and now supports
more recent versions of Xcode than 7, so most of our custom code is
no longer needed.
The xcode packer could also be reworked to use osxcross' SDK packing
script.
Fixes#15.
The previous `['/bin/bash']` was not working, running the image
with `podman run -it <name>` would evaluate to:
```
> sh -c ['/bin/bash']
sh: [/bin/bash]: No such file or directory
```
This saves us from pulling gtk2 and gtk3 as weak deps of
perl and git or whatnot... And we don't need to pull in all
the documentation on Ubuntu.
Also ensures that we properly run `dnf clean all` after each
installation.
