created working cmake build for czmq

I'm not sure of the history of the current CMake build for czmq, but it seems to be safe to say that it was broken (not building and missing some essentials). I rewrote the CMakeLists.txt to handle platform.h checks, pkgconfig stuff, library build, selftest... And tested the new CMakeLists.txt on an Ubuntu system with both libzmq and czmq installed to non-standard directories, and was able to build client applications against both libraries. So this pull should be fully functional.