add warning when ON_INFER is OFF

test=develop

Author: luotao1

CMakeLists.txt

@@ -307,4 +307,7 @@ endif()
 if (ON_INFER)
     message(STATUS "On inference mode, will take place some specific optimization.")
     add_definitions(-DPADDLE_ON_INFERENCE)
+else()
+    #TODO(luotao), combine this warning with `make inference_lib_dist` command.
+    message(WARNING "On inference mode, will take place some specific optimization. Only used in make inference_lib_dist.")
 endif()