patch 8.1.1301: when compiled with VIMDLL some messages are not shown

Problem:    When compiled with VIMDLL some messages are not shown.
Solution:   Set/reset gui.in_use and gui.starting as needed. (Ken Takata,
            closes #4361)

src/main.c

@@ -178,6 +178,8 @@ main
 #ifdef VIMDLL
     // Check if the current executable file is for the GUI subsystem.
     gui.starting = mch_is_gui_executable();
+#elif defined(FEAT_GUI_MSWIN)
+    gui.starting = TRUE;
 #endif
 
 #ifdef FEAT_CLIENTSERVER
@@ -3242,6 +3244,14 @@ mainerr(
     reset_signals();		/* kill us with CTRL-C here, if you like */
 #endif
 
+    // If this is a Windows GUI executable, show an error dialog box.
+#ifdef VIMDLL
+    gui.in_use = mch_is_gui_executable();
+#endif
+#ifdef FEAT_GUI_MSWIN
+    gui.starting = FALSE;   // Needed to show as error.
+#endif
+
     init_longVersion();
     mch_errmsg(longVersion);
     mch_errmsg("\n");