Problem: CMake uses command added in 2.8.12

Solution: bump CMake required version to 2.8.12 to avoid:

CMake Error at tests/CMakeLists.txt:110 (target_include_directories):
  Unknown CMake command "target_include_directories".

CMakeLists.txt

 # CMake build script for ZeroMQ
 
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.12)
 project(ZeroMQ)
 
 list(INSERT CMAKE_MODULE_PATH 0 "${CMAKE_SOURCE_DIR}")