make loses $(src-dir) part of $<

[Graham Percival]

Highly odd make behaviour here.  We seem to have a problem when
running "make doc" inside virtualbox lilydev.  Nobody (including
myself) has seen this problem in their normal lilypond development,
but I can reproduce it occasionally inside lilydev.

I've modified the top of my
  make/doc-i18n-root-rules.make
to be:
$(outdir)/%.texi: $(src-dir)/%.texi
        echo "zzzzzzzzzzzzzzz"
        echo $(src-dir)
        cp -p $< $@

(I added the echos)

My build log now contains:
...
echo "zzzzzzzzzzzzzzz"
zzzzzzzzzzzzzzz
echo /home/lily/lilypond-git/Documentation/es
/home/lily/lilypond-git/Documentation/es
cp -p /home/lily/lilypond-git/Documentation/es/web.texi out-www/web.texi
...
echo "zzzzzzzzzzzzzzz"
zzzzzzzzzzzzzzz
echo /home/lily/lilypond-git/Documentation/fr
/home/lily/lilypond-git/Documentation/fr
cp -p web.texi out-www/web.texi
cp: cannot stat `web.texi': No such file or directory


note that the second cp does *not* include the $(src-dir) portion.

I'm baffled.  I can't see any important difference in the
de/GNUmakefile fr/GNUmakefile es/GNUmakefile... but looking at this
rule, I can't see how any difference in those files would make any
difference.  Something weird is happening between $(src-dir) and $<

Any thoughts?

Cheers,
- Graham