Preferences: disable changing 'U' on blend file versioning

Would have prevented T71612

source/blender/blenloader/intern/readfile.c

@@ -159,6 +159,9 @@
 
 #include <errno.h>
 
+/* Make preferences read-only. */
+#define U (*((const UserDef *)&U))
+
 /**
  * READ
  * ====

source/blender/blenloader/intern/versioning_250.c

@@ -84,6 +84,9 @@
 
 #include <errno.h>
 
+/* Make preferences read-only, use versioning_userdef.c. */
+#define U (*((const UserDef *)&U))
+
 /* 2.50 patch */
 static void area_add_header_region(ScrArea *sa, ListBase *lb)
 {

source/blender/blenloader/intern/versioning_260.c

@@ -79,6 +79,9 @@
 
 #include "readfile.h"
 
+/* Make preferences read-only, use versioning_userdef.c. */
+#define U (*((const UserDef *)&U))
+
 static void do_versions_nodetree_image_default_alpha_output(bNodeTree *ntree)
 {
   bNode *node;

source/blender/blenloader/intern/versioning_270.c

@@ -81,6 +81,9 @@
 
 #include "MEM_guardedalloc.h"
 
+/* Make preferences read-only, use versioning_userdef.c. */
+#define U (*((const UserDef *)&U))
+
 /* ************************************************** */
 /* GP Palettes API (Deprecated) */
 

source/blender/blenloader/intern/versioning_280.c

@@ -96,6 +96,9 @@
 
 #include "MEM_guardedalloc.h"
 
+/* Make preferences read-only, use versioning_userdef.c. */
+#define U (*((const UserDef *)&U))
+
 static bScreen *screen_parent_find(const bScreen *screen)
 {
   /* Can avoid lookup if screen state isn't maximized/full

source/blender/blenloader/intern/versioning_defaults.c

@@ -56,6 +56,9 @@
 
 #include "BLO_readfile.h"
 
+/* Make preferences read-only, use versioning_userdef.c. */
+#define U (*((const UserDef *)&U))
+
 /**
  * Rename if the ID doesn't exist.
  */

source/blender/blenloader/intern/versioning_legacy.c

@@ -89,6 +89,9 @@
 
 #include <errno.h>
 
+/* Make preferences read-only, use versioning_userdef.c. */
+#define U (*((const UserDef *)&U))
+
 static void vcol_to_fcol(Mesh *me)
 {
   MFace *mface;

source/blender/blenloader/intern/writefile.c

@@ -181,6 +181,9 @@
 
 #include <errno.h>
 
+/* Make preferences read-only. */
+#define U (*((const UserDef *)&U))
+
 /* ********* my write, buffered writing with minimum size chunks ************ */
 
 /* Use optimal allocation since blocks of this size are kept in memory for undo. */