It was only defined in the old autotools build to support a >20 year old x11,
https://bugzilla.gnome.org/show_bug.cgi?id=148032
/* Define to the sub-directory where libtool stores uninstalled libraries. */
#mesondefine LT_OBJDIR
-/* Define if <X11/extensions/XIproto.h> needed for xReply */
-#mesondefine NEED_XIPROTO_H_FOR_XREPLY
-
/* Define to 1 if your C compiler doesn't accept -c and -o together. */
#mesondefine NO_MINUS_C_MINUS_O
#include "gdkprivate-x11.h"
#include "gdkdisplay-x11.h"
-#ifdef NEED_XIPROTO_H_FOR_XREPLY
-#include <X11/extensions/XIproto.h>
-#endif
-
#include <X11/Xlibint.h>