stable/2.20 branch does not complete "make doc"

[David Kastrup]

That's not a new development, so there is no point in me to refrain from
cherry-picking further material: the last version of the branch I
checked was from early December and it failed in the same manner.
Compilation of CPU_COUNT=9 make -j9 doc ends with

find ./out-www/web -name '*.html' | xargs grep -L 'UNTRANSLATED NODE: IGNORE 
ME' | sed 's!./out-www/!!g' | xargs 
/usr/local/tmp/lilypond/scripts/build/out/mass-link --prepend-suffix .zh hard 
./out-www /usr/local/tmp/lilypond/Documentation/./out-www
(echo | grep -L 'UNTRANSLATED NODE: IGNORE ME' ) | xargs 
/usr/local/tmp/lilypond/scripts/build/out/mass-link --prepend-suffix .zh hard 
./out-www /usr/local/tmp/lilypond/Documentation/./out-www
true
make[3]: Leaving directory '/usr/local/tmp/lilypond/Documentation/zh'
make[2]: Leaving directory '/usr/local/tmp/lilypond/Documentation'
make[1]: Leaving directory '/usr/local/tmp/lilypond'
make --no-builtin-rules out=www WWW-post
make[1]: Entering directory '/usr/local/tmp/lilypond'
/usr/local/tmp/lilypond/scripts/build/out/www_post LilyPond 2.19.84 ./out-www 
offline
Mirroring...
Traceback (most recent call last):
  File "/usr/local/tmp/lilypond/scripts/build/out/www_post", line 101, in 
<module>
    os.symlink (p, dest)
OSError: [Errno 2] No such file or directory
make[1]: *** [GNUmakefile:162: out-www/offline-root/index.html] Error 1
make[1]: Leaving directory '/usr/local/tmp/lilypond'
make: *** [/usr/local/tmp/lilypond/stepmake/stepmake/generic-targets.make:174: 
doc] Error 2

It's not clear to me what is the problem here and where I would find
logs to figure out more.  The error occurs at a point of time where I
think the main work should already be done and just the offline-root is
getting set up.

I cannot rule out either that VERSION needs to have a particular form
(with regard to VERSION_STABLE and VERSION_DEVEL in relation to the
current version getting compiled) in order to set up things for the web
page, or that a fix already exists in a commit in the master branch that
I just have failed to cherry-pick yet.

If this triggers a bell with anybody, I'd be glad to hear about possible
causes.

Thanks!

-- 
David Kastrup