sane-backends: fix documentation build failure

Disable documentation build, it seems to break with older
texlive/kpathsea versions, fixes:
http://autobuild.buildroot.net/results/44327b543d8918929bf252c5284e8691fda87fd0/


Also switch to alioth mirror since the original tarball is gone from the
regular site when 1.0.23 was released.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>