Native bst support on OS X, remote execution only

As outlined in https://mail.gnome.org/archives/buildstream-list/2018-May/msg00066.html:

Even when the target platform is Linux, there is a large set of developers on either OS X or Windows [laptops]. As we are considering different execution environments on workers, we can also start considering supporting BuildStream natively in these environments.

Remote execution only support is a good starting point.

Some work may be possible to start, but completion will depend on the remote execution client landing in BuildStream #454 (closed)

Some other relevant ML posts:

• https://mail.gnome.org/archives/buildstream-list/2018-June/msg00003.html
• https://mail.gnome.org/archives/buildstream-list/2018-June/msg00006.html
• https://mail.gnome.org/archives/buildstream-list/2018-June/msg00038.html