build: rerun bootstrap if po/Makevars got nuked

build: rerun bootstrap if po/Makevars got nuked

There has been a frequent complaint of:

make[2]: Entering directory `/home/remote/eblake/libvirt/po'
make[2]: *** No rule to make target `/config.status', needed by `Makefile'.  Stop.

It happens after nuking and regenerating the po directory,
which is a common action after running anything like
'make dist' or 'make rpm' that dirties all the .po files.

Teach autogen.sh that it must regenerate po/Makevars to avoid
the missing variable declaration, and teach cfg.mk to recognize
that a nuked po directory is cause to rerun autogen.sh.

* cfg.mk (_update_required): Check for po/Makevars.
* autogen.sh (bootstrap): Run bootstrap if it got lost.
Diagnosed by Justin Clift.