cmake: set default visibility to "hidden"

Currently autotools build uses -fvisibility=hidden, but cmake does not.
This creates a huge ABI difference between the libzmq.so when compiled
with cmake vs autotools. With this patch the difference between the
exported functions is minimum.

Author: stkw0

CMakeLists.txt

@@ -6,6 +6,8 @@ else()
   cmake_minimum_required(VERSION 2.8.12...3.31)
 endif()
 
+set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+
 project(ZeroMQ)
 
 include(CheckIncludeFiles)