We already target the OS X 10.7 API, and 10.7 won't run on machines
that don't have a 64-bit-capable processor. So there's no reason not
to do this now.
By default we build the stable variant but provide a user-editable
list of packages which will be built at the latest variant instead.
Rename chocolate-doom-git package to chocolate-doom and have it use
the latest variant for the current Git build.
