Add internal port/cpl_std_ext.h to emulate C++20 features

port/cpl_std_ext.h

@@ -0,0 +1,45 @@
+#ifndef CPL_STD_EXT_H
+#define CPL_STD_EXT_H
+
+/*! @cond Doxygen_Suppress */
+
+#if defined(GDAL_COMPILATION)
+
+#include <map>
+#include <set>
+
+namespace cpl
+{
+#if __cplusplus > 201703L
+using std::map;
+using std::set;
+#else
+/** Helper offering C++20-like features for std::map */
+template <typename K, typename V> class map : public std::map<K, V>
+{
+  public:
+    inline map() = default;
+    inline bool contains(const K &key) const
+    {
+        return this->find(key) != this->end();
+    }
+};
+
+/** Helper offering C++20-like features for std::set */
+template <typename K> class set : public std::set<K>
+{
+  public:
+    inline set() = default;
+    inline bool contains(const K &key) const
+    {
+        return this->find(key) != this->end();
+    }
+};
+#endif
+}  // namespace cpl
+
+#endif  // GDAL_COMPILATION
+
+/*! @endcond */
+
+#endif  // CPL_STD_EXT_H