When building gotd, also build gotsh.

Compiling -portable on MacOS directly would never have worked, as the environment was only being set when run under CI. Move these checks into configure.ac so that they propagate out to make, etc. Noticed by cjones on IRC.

Remove dependency on libretls as TLS isn't going to be included in gotwebd any time soon. This reverts commit af7f5027866eea7b4299c2319f83d0eb18a395e6.

Enable CI builds for MaxcOS on CirrusCI.

Cirrus-CI allows for FreeBSD builds as well as Linux.