Opening this issue to hopefully save somebody else a lot of frustration:
The git HEAD version of gitian-builder uses lxc-start --version which is a new option that doesn't exist in the version of LXC you get with Debian 7; specific error is:
$ make-clean-vm --suite precise --arch amd64
lxc-start: unrecognized option '--version'
The doc/gitian-building.md instructions should either be updated to recommend using a specific commit of gitian-builder (2bcc06e6b75b3f7a0167cde7237331757dd559c6 worked for me), or somebody should work through using Debian 8 as the host operating system (I started down that path but don't understand LXC deeply enough to figure out how to make it all work).