sm3600.h: Fix build with standard C23

bool, true and false are keywords in the standard C23 - the sm3600
backend defines them on its own, which is forbidden with the new
standard.

The patch adds ifdef guards for the affected typedef - the old code
will be used for older standards, C23 will define TBool as bool.

backend/sm3600.h

@@ -77,7 +77,11 @@ Start: 2.4.2001
 
 /* ====================================================================== */
 
+#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
 typedef enum { false, true } TBool;
+#else
+typedef bool TBool;
+#endif /* GCC < 15 */
 
 typedef SANE_Status TState;