For those who like to use centos built-in zlib which I assume is sercurity fixed and could be updated easily with yum, I have used a simple workaround.
Before installing ISPConfig,
have a look at /install_ispconfig/compile_aps/compile and search for cd $(CLAMAV) at about line 157.
At the next line add --disable-zlib-vcheck after ./configure.
ISPConfig should than compile with centos original zlib.