Udpate unittests/CMakeLists.txt: use ZeroMQ_SOURCE_DIR, ZeroMQ_BINARY_DIR

Use ZeroMQ_SOURCE_DIR, ZeroMQ_BINARY_DIR instead of CMAKE_SOURCE_DIR, CMAKE_BINARY_DIR, which allows libzmq to become an "add_subdirectory" target (with static build).
Otherwise, building unittest_xxx would complain header files not found.

unittests/CMakeLists.txt

@@ -22,8 +22,8 @@ if(WIN32)
     link_libraries(ws2_32.lib)
 endif()
 
-include_directories("${CMAKE_SOURCE_DIR}/include" "${CMAKE_SOURCE_DIR}/src" "${CMAKE_BINARY_DIR}")
-include_directories("${CMAKE_SOURCE_DIR}/external/unity")
+include_directories("${ZeroMQ_SOURCE_DIR}/include" "${ZeroMQ_SOURCE_DIR}/src" "${ZeroMQ_BINARY_DIR}")
+include_directories("${ZeroMQ_SOURCE_DIR}/external/unity")
 
 foreach(test ${unittests})
   # target_sources not supported before CMake 3.1
@@ -31,7 +31,7 @@ foreach(test ${unittests})
 
   # per-test directories not generated on OS X / Darwin
   if (NOT ${CMAKE_CXX_COMPILER_ID} MATCHES "Clang.*")
-      link_directories(${test} PRIVATE "${CMAKE_SOURCE_DIR}/../lib")     
+      link_directories(${test} PRIVATE "${ZeroMQ_SOURCE_DIR}/../lib")     
   endif()
   
   target_link_libraries(${test} libzmq-static ${OPTIONAL_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} unity)