cmake: Add zlib dependency to CMakeLists.txt

When building on fedora-18 xfce, it does not
prompt a warning that I don't have zlib-devel.

But it will appear at make: can't find zlib.h.
Merge pull request #312 from ShengYun/bugfix-missed-dependency-in-cmake

cmake: Add zlib dependency to CMakeLists.txt
Merge branch 'upstream'