Don't depend on acroread-5, because it is forbidden.

Note: acroread-5 was used to provide the font Symbol, which is not
installed by acroread-7, then users will get messages from mozilla
complaining that there's no Symbol font; anyway, the MathML pages
should be displayed correctly, because the missing characters can be
found in the other fonts.

Suggested by: Yann Golanski <yg2 (at) york.ac.uk>