Use appropriate macro.

plugins/mongo_db_plugin/CMakeLists.txt

@@ -38,7 +38,7 @@ if(BUILD_MONGO_DB_PLUGIN)
 
       # mongo-cxx-driver 3.2 release altered LIBBSONCXX_LIBRARIES semantics. Instead of library names,
       #  it now hold library paths.
-      if((LIBBSONCXX_VERSION_MAJOR LESS 3) OR ((LIBBSONCXX_VERSION_MAJOR EQUAL 3) AND (LIBBSONCXX_VERSION_MINOR LESS 2)))
+      if((LIBMONGOCXX_VERSION_MAJOR LESS 3) OR ((LIBMONGOCXX_VERSION_MAJOR EQUAL 3) AND (LIBMONGOCXX_VERSION_MINOR LESS 2)))
         find_library(EOS_LIBMONGOCXX ${LIBMONGOCXX_LIBRARIES}
           PATHS ${LIBMONGOCXX_LIBRARY_DIRS} NO_DEFAULT_PATH)
       else()