Fix compilation: tests/async_queries links againist client library, and

must use C++ linking due to client library using SSL library, which needs
C++ linking

tests/CMakeLists.txt

@@ -34,4 +34,6 @@ FIND_LIBRARY(EVENT_LIBRARY event)
 IF(HAVE_EVENT_H AND EVENT_LIBRARY)
   ADD_EXECUTABLE(async_queries async_queries.c)
   TARGET_LINK_LIBRARIES(async_queries mysqlclient ${EVENT_LIBRARY})
+  SET_TARGET_PROPERTIES(async_queries PROPERTIES LINKER_LANGUAGE CXX)
+
 ENDIF()