Build: Small dependency fix (issue 3442).

Build: Small dependency fix (issue 3442).

This sets the dependency of texi files more accurately and it rids
us of strange errors like the following from building the zh doc:

  Must remake target `web.texi'.
  Ignoring VPATH name `~/git/lilypond/Documentation/zh/web.texi'.
  Successfully remade target file `web.texi'.
 [...]
 Finished prerequisites of target file `out-www/web.texi'.
Must remake target `out-www/web.texi'.
mkdir -p out-www/
( echo ./out-www/web.texi:  > out-www/web.dep ) && cp -f web.texi out-www/web.texi
cp: cannot stat `web.texi': No such file or directory
make[1]: *** [out-www/web.texi] Error 1