Suppress mpd_setminalloc warning

Modules/_decimal/_decimal.c

@@ -5898,7 +5898,13 @@ _decimal_exec(PyObject *m)
     mpd_reallocfunc = PyMem_Realloc;
     mpd_callocfunc = mpd_callocfunc_em;
     mpd_free = PyMem_Free;
-    mpd_setminalloc(_Py_DEC_MINALLOC);
+
+    /* Suppress the warning caused by multi-phase initialization */
+    static int minalloc_is_set = 0;
+    if (minalloc_is_set) {
+        mpd_setminalloc(_Py_DEC_MINALLOC);
+        minalloc_is_set = 1;
+    }
 
     decimal_state *state = get_module_state(m);