Advance the minimum required version of "flex" from 2.5.4 to 2.5.31, and

Advance the minimum required version of "flex" from 2.5.4 to 2.5.31, and
update documentation accordingly.  This is required in order to have support
for a reentrant scanner.  I'm committing this bit separately in order to have
an easy reference if we later decide to make the minimum something different
(like 2.5.33).