Add support for automatically updating Unicode derived files

Continuing the discussion from [0] and [1], here is a patch that
automates the process of updating Unicode derived files. Summary:

- Edit UNICODE_VERSION and/or CLDR_VERSION in src/Makefile.global.in
- Run make update-unicode
- Commit

I have added that to the release checklist in RELEASE_NOTES.

This also includes the script used in [0] that was not committed at that
time. Other than that, this just refactors existing build code.

Open questions that are currently not handled consistently:

- Should the downloaded files be listed in .gitignore?
- Should the downloaded files be cleaned by make clean (or distclean or
maintainer-clean or none)?
- Should the generated files be excluded from pgindent? Currently, the
generated files will not pass pgindent unchanged, so that could cause
annoying whitespace battles when these files are updated and re-indented
around release time.

[0]:
https://www.postgresql.org/message-id/flat/bbb19114-af1e-513b-08a9-61272794bd5c%402ndquadrant.com
[1]:
https://www.postgresql.org/message-id/flat/77f69366-ca31-6437-079f-47fce69bae1b%402ndquadrant.com

--
Peter Eisentraut http://www.2ndQuadrant.com/
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