CMakeLists: Add zlib as a dependency

gcc and clang complain "undefined reference to (zlib)".
To solve this, add zlib as a dependency, and link it with dragonscurse
Merge branch 'master' of git://github.com/phoboz/dragonscurse
Merge pull request #1 from crumblingstatue/master

CMakeLists: Add zlib as a dependency