Do not reference SecureAllocator when not JSONCPP_USING_SECURE_MEMORY

Amalgamated version of jsoncpp does not include allocator.h and as a result doesn't have definition of SecureAllocator and results in compilation error when referencing SecureAllocator in config.h

Author: pps83

Diff for: include/json/config.h

@@ -173,11 +173,11 @@ typedef UInt64 LargestUInt;
 #define JSON_HAS_INT64
 #endif // if defined(JSON_NO_INT64)
 
-template <typename T>
-using Allocator = typename std::conditional<JSONCPP_USING_SECURE_MEMORY,
-                                            SecureAllocator<T>,
-                                            std::allocator<T>>::type;
-using String = std::basic_string<char, std::char_traits<char>, Allocator<char>>;
+#if JSONCPP_USING_SECURE_MEMORY
+using String = std::basic_string<char, std::char_traits<char>, SecureAllocator<char>>;
+#else  // !JSONCPP_USING_SECURE_MEMORY
+using String = std::string;
+#endif // JSONCPP_USING_SECURE_MEMORY
 using IStringStream = std::basic_istringstream<String::value_type,
                                                String::traits_type,
                                                String::allocator_type>;