Cleanup: use static assert to disable access to preferences

Prefer this over an unknown identifier as it gives a useful error.

source/blender/blenkernel/intern/blender.c

@@ -260,8 +260,8 @@ static void userdef_free_addons(UserDef *userdef)
  */
 void BKE_blender_userdef_data_free(UserDef *userdef, bool clear_fonts)
 {
-#define U _invalid_access_ /* ensure no accidental global access */
-#ifdef U                   /* quiet warning */
+#define U BLI_STATIC_ASSERT(false, "Global 'U' not allowed, only use arguments passed in!")
+#ifdef U /* quiet warning */
 #endif
 
   userdef_free_keymaps(userdef);

source/blender/blenloader/intern/versioning_userdef.c

@@ -48,8 +48,8 @@
 
 #include "wm_event_types.h"
 
-/* Disallow access to global userdef. */
-#define U (_error_)
+/* For versioning we only ever want to manipulate preferences passed in. */
+#define U BLI_STATIC_ASSERT(false, "Global 'U' not allowed, only use arguments passed in!")
 
 static void do_versions_theme(const UserDef *userdef, bTheme *btheme)
 {