gd: Add mirror for when libgd.org is down

Added http://fossies.org/unix/www/ as a mirror of http://www.libgd.org/releases/
so that gd-2.0.36RC1.tar.gz can still be downloaded when libgd.org is down.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>