Buildbot: create a new factory to aid with new MacPort releases

[mojca (Mojca Miklavec)]

It would be helpful to have a new buildbot factory to create everything needed for a new release, other than messing up with the repository (tagging, updating the docs, ...) and signing the archives.

Creating a new release is a bit problematic because it requires access to:

• all the supported OS versions
• payed developer account to sign the packages

We cannot avoid the need to sign the packages (unless the server can do that automatically, but even then it might be a security concern), but we could use the existing infrastructure to compile the components of MacPorts on the target architecture and create the unsigned pkg/dmg files. Well, we would still need a virtual machine for Tiger and Leopard, but 6-7 (or more if we also count the libc++-based builds) could be catered for by the existing infrastructure.

[ryandesign (Ryan Carsten Schmidt)]

Yes, we should automate building and posting the official binaries of base when a release tag is created.

Base doesn't use C++ so we don't need to worry about the libstdc++ / libc++ distinction. That's why I've only set up one base builder per macOS version.