Support for building mac keyboards on Linux

[ftyers]

$ python3 -m kbdgen -t osx examples/project-cux.yaml 
[E osx:68] [cuicatec] Error while validating layout:
    Dead key modes are defined for undefined modes: ['iso-default', 'iso-shift']
[I osx:79] Creating bundle…
[I osx:100] Creating installer…
[W osx:313] No version for installer specified; defaulting to '0.0.0'.
[E base:222] Process failed to launch with the following error message:
[E base:223] [Errno 2] No such file or directory: 'pkgbuild': 'pkgbuild'

It seems likepkgbuildis something on Mac that isn't on Linux. It seems to be possible (in principle).