X11: avoid null-pointer-subtraction warnings

Most recent version of XQuartz, same as before.  Unfortunately,
newer version of macOS => newer version of Xcode and its command
line tools => newer version of clang => emulating newer version of
gcc which defaults to a more recent version of StdC, I suppose, or
perhaps our hints are specifying that.  Whichever, it has resulted
in a bunch of complaints about XtOffset() used in win/X11/winX.c:
|warning: performing pointer subtraction with a null pointer has\
 undefined behavior [-Wnull-pointer-subtraction]

Adding -wno-null-pointer-subtraction to X11FLAGS silences them,
but that would require figuring out which versions of gcc and
clang added -Wnull-pointer-subtraction and its negation.  Revising
XtOffset() to include the ptrdiff_t casts eliminates the warnings,
avoiding the need for version conditionals to deal with X11FLAGS.

include/winX.h

@@ -1,4 +1,4 @@
-/* NetHack 3.7	winX.h	$NHDT-Date: 1643491525 2022/01/29 21:25:25 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.52 $ */
+/* NetHack 3.7	winX.h	$NHDT-Date: 1740795096 2025/02/28 18:11:36 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.65 $ */
 /* Copyright (c) Dean Luick, 1992                                 */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -26,6 +26,19 @@
 #endif
 #endif
 
+/* winX.c uses XtOffset() and the way that that macro is defined in
+   <X11/Intrinsic.h> triggers "performing pointer subtraction with
+   a null pointer has undefined behavior" warnings; this modified
+   edition doesn't guarantee defined behavior but does silence those
+   warnings without needing to know whether current compiler version
+   supports the '-wno-null-pointer-subtraction' option */
+#ifdef XtOffset
+#undef XtOffset
+#define XtOffset(p_type,field) \
+    ((Cardinal) (((ptrdiff_t) (char *) (&(((p_type) NULL)->field)))     \
+                 - ((ptrdiff_t) (char *) NULL)))
+#endif
+
 /*
  * Generic text buffer.
  */