Add SPDLOG_TO_VERSION to compare spdlog version

You can use SPDLOG_VERSION to select the latest spdlog features
where available while falling back to older implementations otherwise.
Using SPDLOG_TO_VERSION() for the value to compare with is recommended.
for Example:
```c++
 void sink_it_(const details::log_msg &msg) override
 {
 #if SPDLOG_VERSION < SPDLOG_TO_VERSION(1,4,0)
     fmt::memory_buffer formatted;
 #else
     memory_buf_t formatted;
 #endif
     sinks::base_sink<Mutex>::formatter_->format(msg, formatted);
     // bala bala...
 }
```

include/spdlog/version.h

@@ -7,4 +7,5 @@
 #define SPDLOG_VER_MINOR 12
 #define SPDLOG_VER_PATCH 0
 
-#define SPDLOG_VERSION (SPDLOG_VER_MAJOR * 10000 + SPDLOG_VER_MINOR * 100 + SPDLOG_VER_PATCH)
+#define SPDLOG_TO_VERSION(major, minor, patch) (major * 10000 + minor * 100 + patch)
+#define SPDLOG_VERSION SPDLOG_TO_VERSION(SPDLOG_VER_MAJOR, SPDLOG_VER_MINOR, SPDLOG_VER_PATCH)