portable: ci: add ubuntu Add Ubuntu as a dedicated CI environment. By default, sh is aliase to dash which often provides interesting challenges for portability, so making this work here is a good thing as mistakes will be spotted earlier.

portable: alpine: install bsd-compat-headers Apline doesn't like the use of cdefs.h

CI: add alpine linux Add Alpine Linux to the list of CI checks. Alpine builds against muscl rather than glibc by default, and there's been a few compilation differenes to warrant having this distribution as a separate CI check.

portable: CI: add MacOS definition Enable CI builds for MaxcOS on CirrusCI.

Cirrus-CI: add build YAML file Cirrus-CI allows for FreeBSD builds as well as Linux.