Our build environment predefines __EXTENSIONS__, so patch the places
that define it to tolerate the predefine.

--- gnucash-5.11/libgnucash/engine/gnc-date.cpp.~1~	Mon Nov 11 18:02:45 2024
+++ gnucash-5.11/libgnucash/engine/gnc-date.cpp	Wed Apr 30 13:58:50 2025
@@ -25,7 +25,9 @@
  *                                                                  *
 \********************************************************************/
 
+#ifndef __EXTENSIONS__
 #define __EXTENSIONS__
+#endif
 #include <glib.h>
 
 #include <config.h>
--- gnucash-5.11/libgnucash/backend/xml/sixtp-utils.cpp.~1~	Mon Nov 11 18:02:45 2024
+++ gnucash-5.11/libgnucash/backend/xml/sixtp-utils.cpp	Wed Apr 30 14:32:53 2025
@@ -21,7 +21,9 @@
  *                                                                  *
  ********************************************************************/
 
+#ifndef __EXTENSIONS__
 #define __EXTENSIONS__
+#endif
 #include <guid.hpp>
 #include <config.h>
 
--- gnucash-5.11/libgnucash/backend/xml/sixtp-dom-generators.cpp.~1~	Mon Nov 11 18:02:45 2024
+++ gnucash-5.11/libgnucash/backend/xml/sixtp-dom-generators.cpp	Wed Apr 30 14:33:12 2025
@@ -22,7 +22,9 @@
  ********************************************************************/
 #include <glib.h>
 
+#ifndef __EXTENSIONS__
 #define __EXTENSIONS__
+#endif
 
 #include <config.h>