4.9 Build system

Version-specific Texinfo macros

• made with scripts/build/create-version-itexi.py and
  scripts/build/create-weblinks-itexi.py
• used extensively in the WEBSITE_ONLY_BUILD version of the website (made with website.make, used on lilypond.org)
• not (?) used in the main docs?
• the numbers in VERSION file: MINOR_VERSION should be 1 more than the last release, VERSION_DEVEL should be the last online release. Yes, VERSION_DEVEL is less than VERSION.