portable: CI (linux): add libbsd-dev This needs to be explicitly installed.

portable: improve macos compat with bison/openssl Some MacOS systems might not have GNU Bison installed, or have openssl installed. In such instances the host versions of those can be used, with a few tweaks around the edges to make them work. This commit addresses these by: * Relaxing a HAVE_CRYPTO check for __APPLE__, since the inclusion of the machine-specific headers isn't dependent on openssl being installed; * If the non-GNU version of bison on MacOS is being used, define any missing YY* variables, and add a compiler #warning so we log something. This also updates the CI scripts to default to the system versions of bison and removes openssl as a core dependency -- it's more likely openssl is going to be installed, so removing it here will mean we can catch the case for just the MacOS-specific failures, and the inclusion of openssl on other OSes should cover the rest. Noticed by Evan Silberman

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.