fix dynamic flag bug on mac (#41571) (#41660)

cherry-pick #41571

Author: zhwesky2010

paddle/fluid/platform/flags.cc

@@ -535,6 +535,11 @@ PADDLE_DEFINE_EXPORTED_double(
     "you should set FLAGS_local_exe_sub_scope_limit=-1. "
     "The default value is 256 MBytes.");
 
+PADDLE_DEFINE_EXPORTED_bool(
+    reader_queue_speed_test_mode, false,
+    "If set true, the queue.pop will only get data from queue but not "
+    "remove the data from queue for speed testing");
+
 /**
  * MKLDNN related FLAG
  * Name: use_mkldnn

paddle/fluid/pybind/reader_py.cc

@@ -32,10 +32,7 @@
 #include "paddle/phi/core/ddim.h"
 #include "pybind11/stl.h"
 
-PADDLE_DEFINE_EXPORTED_bool(
-    reader_queue_speed_test_mode, false,
-    "If set true, the queue.pop will only get data from queue but not "
-    "remove the data from queue for speed testing");
+DECLARE_bool(reader_queue_speed_test_mode);
 
 // disable auto conversion to list in Python
 PYBIND11_MAKE_OPAQUE(paddle::framework::LoDTensorArray);