Only build _version.py once; remove phony targets

Only build _version.py once; remove phony targets

Previously _version.py was a phony target because we couldn't easily
tell when the current git working tree version had changed.  This caused
various targets to be rebuilt multiple times (i.e. recursive make
invocations, etc.).

To fix that, just update _version.py once (at startup) if needed, via an
immediate variable assignment that calls a new ./configure-version
command, i.e.

  initial_setup := $(shell ./configure-version --update)

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>