The tar archives generated by gitian are currently unversioned. This
adds either a tag name when building from a tag, or a short commit id
when building from a commit hash.
Building with docker is arguably easier and more familiar to most people
than either kvm, or lxc.
This commit also relaxes the back compat requirement a bit. 32 bit linux
now uses glibc version 2.0. Also, the docker shell could not handle gcc arguments
containing spaces, so the explicit '-DFELT_TYPE' declaration was dropped.
Lastly, this removes some packages from the osx descriptor.
