Kleopatra: port to libassuan v2

This is a clean cut. At least libassuan v2.0.0 is required (it's the first
one that can be built into a DSO, which is the only sane way to deal with
libraries...). However, libassuan isn't required. If not found, Kleopatra
should be built with reduced functionality, as before.

svn path=/branches/kdepim/enterprise4/kdepim/; revision=1074705