Support C++20 std::format as an alternative to fmtlib

include/spdlog/sinks/msvc_sink.h

@@ -30,7 +30,11 @@ protected:
     {
         memory_buf_t formatted;
         base_sink<Mutex>::formatter_->format(msg, formatted);
+#ifdef SPDLOG_USE_STD_FORMAT
+        OutputDebugStringA(formatted.c_str());
+#else
         OutputDebugStringA(fmt::to_string(formatted).c_str());
+#endif
     }
 
     void flush_() override {}