Do not redefine bool constants in C23

C23 now includes predefined constants for the boolean values true and
false, and attempting to redefine them ends with a compiler error. Avoid
doing that by adjusting the preprocessor guard around that.

Upstream-Status: Submitted [email]
--- include/sys-defines.h.orig	2026-04-13 22:50:46
+++ include/sys-defines.h	2026-04-13 22:53:57
@@ -253,7 +253,7 @@
 /* Support the `bool' datatype, which is used widely in this package.     */
 /**************************************************************************/
 
-#ifndef __cplusplus
+#if !defined(__cplusplus) && (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L)
 #ifdef __STDC__
 typedef enum { false = 0, true = 1 } bool;
 #else  /* not __STDC__, do things the old-fashioned way */
@@ -261,7 +261,7 @@
 #define false 0
 #define true 1
 #endif /* not __STDC__ */
-#endif /* not __cplusplus */
+#endif /* not __cplusplus and (not __STDC_VERSION__ or __STDC_VERSION__ < 202311L) */
 
 /**************************************************************************/
 /* Define numerical constants (unofficial, so may not be in math.h).      */