Update to ZMQ4 and support older processors without POPCNT instruction. 

Older processors without POPCNT are now detected at cmake and SSSE3 implementation of POPCNT is used instead. Since currently we use SSE3 intrinsics explicitly in the vector ops SSSE3 is absolute minimum but we cannot yet take advantage of AVX. Watch this space.

ZMQ support has now gone to ZMQ4 with this version and seems to work (up to unit tests) at least on Linux platforms.