We currently have several sets of files generated from data provided
by Unicode.  These all have ad hoc rules and instructions for updating
when new Unicode versions appear, and it's not done consistently.

This patch centralizes and automates the process and makes it part of
the release checklist.  The Unicode and CLDR versions are specified in
Makefile.global.in.  There is a new make target "update-unicode" that
downloads all the relevant files and runs the generation script.

There is also a new script for generating the table of combining
characters for ucs_wcwidth().  That table is now in a separate include
file rather than hardcoded into the middle of other code.  This is
based on the script that was used for generating
d8594d12, but the script itself wasn't
committed at that time.
Reviewed-by: John Naylor <john.naylor@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/c8d05f42-443e-6c23-819b-05b31759a37c@2ndquadrant.com