Remove logic for wgeting the latest config.guess and config.sub.

This logic is from long ago, when the GNU Autotools releases were few
and far between.  Now, the Autotools publish new versions
semi-regularly, and new platforms are more rare.  So we should be fine
using the config.guess and config.sub that are in official Autotools
releases.
Don't have make_snapshot_tarball insert "git describe" value in VERSION.

Instead, just ensre that VERSION is set with
snapshot=1/snapshot_version= (i.e., empty).  With these settings,
config/distscript.csh will fill in snapshot_version with an
appropriate value (currently: git describe output) and make a
properly-named snapshot tarball.