compatible for c++11 but GCC < 5

include/spdlog/fmt/bundled/format.h

@@ -72,6 +72,12 @@
 #  endif
 #endif  // FMT_MODULE
 
+#if (defined(__GNUC__) && __GNUC__ < 5) || defined(_MSC_VER)
+#  define OPERATOR_LITERAL(suffix) operator"" suffix
+#else
+#  define OPERATOR_LITERAL(suffix) operator""suffix
+#endif
+
 #if defined(FMT_USE_NONTYPE_TEMPLATE_ARGS)
 // Use the provided definition.
 #elif defined(__NVCOMPILER)
@@ -3993,7 +3999,7 @@ template <typename T, typename Char = char> struct nested_formatter {
 
 inline namespace literals {
 #if FMT_USE_NONTYPE_TEMPLATE_ARGS
-template <detail::fixed_string S> constexpr auto operator""_a() {
+template <detail::fixed_string S> constexpr auto OPERATOR_LITERAL(_a)() {
   using char_t = remove_cvref_t<decltype(*S.data)>;
   return detail::udl_arg<char_t, sizeof(S.data) / sizeof(char_t), S>();
 }
@@ -4006,7 +4012,7 @@ template <detail::fixed_string S> constexpr auto operator""_a() {
  *     using namespace fmt::literals;
  *     fmt::print("The answer is {answer}.", "answer"_a=42);
  */
-constexpr auto operator""_a(const char* s, size_t) -> detail::udl_arg<char> {
+constexpr auto OPERATOR_LITERAL(_a)(const char* s, size_t) -> detail::udl_arg<char> {
   return {s};
 }
 #endif  // FMT_USE_NONTYPE_TEMPLATE_ARGS