ZLIB not being checked

[Paulo Jorge Guedes]

Hi,

kgzipfilter.cpp is not compiled if zlib is not found.
This causes a linkage error because the pieces of code that use gzip are
not surrounded by some macro like bzip2 for example (#ifdef
HAVE_BZIP2_SUPPORT).
I can see that there is already a "HAVE_LIBZ" being set, although the
code doesn't use it.
If you are interested in such a patch please let me know. 

Paulo