Sim/Export/MaterialKeyHandler: implement a C++17 version for std::map::contains method (C++20)

Sim/Export/MaterialKeyHandler.cpp

@@ -26,7 +26,14 @@ void MaterialKeyHandler::insertMaterial(const Material* mat)
     m_Mat2Unique.emplace(mat, mat);
 
     const std::string key = "material_" + mat->materialName();
-    ASSERT(!m_Key2Mat.contains(key)); // material must not be exported more than once
+
+    // material must not be exported more than once
+#ifdef BA_CPP20
+    ASSERT(!m_Key2Mat.contains(key));
+#else
+    ASSERT(m_Key2Mat.find(key) == m_Key2Mat.end());
+#endif // BA_CPP20
+
     m_Key2Mat.emplace(key, mat);
 }