out-of-tree fr doc build failure

[Graham Percival]

writing: /main/src/build-lilypond/./out-www/xref-maps/usage.fr.xref-map
touch -r out-www/usage.texi
/main/src/build-lilypond/./out-www/xref-maps/usage.fr.xref-map
cp -p web.texi out-www/web.texi
cp: cannot stat `web.texi': No such file or directory
make[3]: *** [out-www/web.texi] Error 1
make[3]: Leaving directory `/main/src/build-lilypond/Documentation/fr'
make[2]: *** [WWW-1] Error 2
make[2]: Leaving directory `/main/src/build-lilypond/Documentation'


I'm guessing that we're missing a $(top-src-dir) somewhere, but I'm at
the end of my 3 hours today.

Cheers,
- Graham