build: only use flat_set when available

GCC 14 does not have flat_set despite -std=c++23 being accepted.

Author: jengelh

include/gromox/flat_set.hpp

@@ -1,5 +1,7 @@
 #pragma once
-#if __cplusplus >= 202300L
+#if __cplusplus >= 202000L
+#include <version>
+#ifdef __cpp_lib_flat_set
 #include <flat_set>
 namespace gromox {
 template<typename T> using maybe_flat_set = std::flat_set<T>;
@@ -10,3 +12,4 @@ namespace gromox {
 template<typename T> using maybe_flat_set = std::set<T>;
 }
 #endif
+#endif