Rewrite updateVersions script in Python, add datestamp functionality.

This updateVersions.pl script was one of the only essential perl
scripts left in out maint system, and was the only one that used
autoconf to fill in the script.

This script adds a feature to define an APPROX_RELEASE_DATE macro
that is updated when the version changes.  We'll use this to
implement prop297, so that we have an accurate view of when a
release date happens.