GUB: libtiff missing from lilypond bootstrap

[Graham Percival]

After running
    make -f lilypond.make bootstrap
there's nothing in downloads/libtiff/, causing
    make lilypond
to fail:


*** Stage: untar (libtiff, tools)
invoking rm -rf /home/lilypond/gub/target/tools/src/libtiff-3.8.2
/home/lilypond/gub/target/tools/build/libtiff-3.8.2
/home/lilypond/gub/target/tools/install/libtiff-3.8.2-root
invoking mkdir /home/lilypond/gub/target/tools/src/libtiff-3.8.2
invoking tar -C /home/lilypond/gub/target/tools/src/libtiff-3.8.2
--strip-component=1 -v -z -xf
/home/lilypond/gub/downloads/libtiff/tiff-3.8.2.tar.gz
tar: /home/lilypond/gub/downloads/libtiff/tiff-3.8.2.tar.gz:
Cannot open: No such file or directory
tar: Error is not recoverable: exiting now


This is easy fixed by manually calling
    bin/gub tools::libtiff
but it would be nice if the dependency was set up in the
bootstrap.

Cheers,
- Graham