`s9pk.mk` is the shared Start9 build logic. It's a boilerplate file that
the Start9 team maintains — every 0.4.0.x package ships a copy of it that
hardly changes between services.

To add it to this project:

    curl -o s9pk.mk https://raw.githubusercontent.com/Start9Labs/hello-world-startos/master/s9pk.mk

Or clone the `hello-world-startos` repo and copy the file over. The canonical
source is whatever the Start9 team maintains in that template.

Reason we don't commit a copy here by default: the file is updated by the
Start9 team as the SDK evolves, and pinning a copy means we'd fall behind.
The Makefile `include s9pk.mk` line will fail loudly if it's missing,
reminding you to fetch it.