Improve optional/default squashing.

Improve optional/default squashing.

I've verified that:
  - OSX/Darwin-specific docker-squash,
  - with gnu-tar 1.29 (>=1.27) in the path as "tar",
  - and run as root,

actually works.  As such, using docker-squash is now the default
behavior (consistent with the previous behavior), but you can opt out of
it by prefix your `make` command with the `SQUASH=NO` environment
variable:
    SQUASH=NO make all

Also note that rather than running the _entire_ make with "sudo", it's
now used specifically for the docker-squash call only, so that the
various make commands aren't running more privileged than they really
need to.