deterministically build crypt.pdf

This entails:

 • Using pdflatex to influence the modification/creation timestamp in
   the resulting PDF
 • Replacing the unique ID with 0/0
 • Using the timestamp from crypt.tex instead of the current date/time

See also http://superuser.com/a/130804

This change is necessary because for Multi-Arch Debian packages,
non-arch-dependent files need to have the same checksum, which is not
guaranteed without this commit. See http://bugs.debian.org/734109