Add JSON_NO_UDLS to leave out the user-defined string literals

The bodies of operator""_json and operator""_json_pointer call the
parser, so every translation unit including the library instantiates it,
even if it never parses anything. Defining JSON_NO_UDLS leaves the
literals out entirely, which saves 15-35% compile time for such
translation units (#5294). Nothing changes if the macro is not defined.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

src/modules/json.cppm

@@ -32,6 +32,7 @@ using NLOHMANN_JSON_NAMESPACE::ordered_json;
 using NLOHMANN_JSON_NAMESPACE::ordered_map;
 using NLOHMANN_JSON_NAMESPACE::to_string;
 
+#ifndef JSON_NO_UDLS
 inline namespace literals
 {
 inline namespace json_literals
@@ -40,6 +41,7 @@ inline namespace json_literals
     using NLOHMANN_JSON_NAMESPACE::literals::json_literals::operator""_json_pointer;
 } // namespace json_literals
 } // namespace literals
+#endif
 
 // Note: the following nlohmann::detail symbols must be exported due to
 // an MSVC bug failing to compile without these symbols visible (ticket #3970)