fix gui mode detection, use the same redirect-output test

git-svn-id: https://xpra.org/svn/Xpra/trunk@15725 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/platform/win32/__init__.py

@@ -292,10 +292,9 @@ def do_init():
 def _show_message(message, uType):
     global prg_name
     #TODO: detect cx_freeze equivallent
-    GUI_MODE = hasattr(sys, "frozen") and sys.frozen=="windows_exe"
     from xpra.util import envbool
     SHOW_MESSAGEBOX = envbool("XPRA_MESSAGEBOX", True)
-    if SHOW_MESSAGEBOX and GUI_MODE:
+    if SHOW_MESSAGEBOX and REDIRECT_OUTPUT:
         #try to use an alert box since no console output will be shown:
         try:
             MessageBoxA(0, message, prg_name, uType)