Conditionally remove unreachable for C23+

include/jemalloc/internal/util.h

@@ -39,6 +39,15 @@
 #  define unlikely(x) !!(x)
 #endif
 
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L
+#include <stddef.h>
+#else
+#if !defined(JEMALLOC_INTERNAL_UNREACHABLE)
+#  error JEMALLOC_INTERNAL_UNREACHABLE should have been defined by configure
+#endif
+#define unreachable() JEMALLOC_INTERNAL_UNREACHABLE()
+#endif
+
 /* Set error code. */
 UTIL_INLINE void
 set_errno(int errnum) {