Problem: CMake on Linux does not link libzmq with optional libs

Solution: add missing target_link_libraries
Fixes #2701

CMakeLists.txt

@@ -776,6 +776,7 @@ if (MSVC)
 else ()
   if (BUILD_SHARED)
     add_library (libzmq SHARED ${sources} ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig})
+    target_link_libraries (libzmq ${OPTIONAL_LIBRARIES})
     # NOTE: the SOVERSION MUST be the same as the one generated by libtool!
     set_target_properties (libzmq PROPERTIES
                           COMPILE_DEFINITIONS "DLL_EXPORT"