[S] Change in flashrom[main]: Makefile,meson.build: Add support for Sphinx 3.x

[Anton Samsonov (Code Review)]

Attention is currently required from: Anastasia Klimchuk, Anton Samsonov, Thomas Heijligen.

Anton Samsonov has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/77778?usp=email )

Change subject: Makefile,meson.build: Add support for Sphinx 3.x
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Patch Set 5:

(1 comment)

Commit Message:

https://review.coreboot.org/c/flashrom/+/77778/comment/fb09dd14_4d0e0047 :
PS3, Line 13: but not on `sphinx-build` own level
            : upon which `meson` build relies
> Alternatively, a symlink "8" may be created pointing at "man8" after the directory renaming occurs. In that case `sphinx-build` 3.x would be able to avoid unnecessary rebuilds.

Here we go. But `sphinx-build` actually rebuilds the man page each time it is run — unlike stated in doc/meson.build comments regarding `build_always_stale` (in fact, this option lives up to its name).