use stable repo for libzmq
Merge pull request #354 from mhamrle/use_stable_zmq

Use stable repo for libzmq
Mark parameters that could not be used

In android build configuration some function params is not used but they are used in other builds. This commit adds new macro ATTR_UNUSED that on gcc and clang supress warning.
callback is not used in android build
fix print format
fix old style cast