Fix Visual Studio poor std::this_thread::get_id() performance by using GetCurrentThreadId() (and pthread_self() under linux)

include/spdlog/details/os.h

@@ -32,6 +32,8 @@
 #  define WIN32_LEAN_AND_MEAN
 # endif
 # include <Windows.h>
+#else
+#include <pthread.h>
 #endif
 
 #include "../common.h"
@@ -167,6 +169,17 @@ inline int utc_minutes_offset(const std::tm& tm = details::os::localtime())
 }
 
 
+inline uint64_t thread_id()
+{
+
+#ifdef _WIN32
+    return ::GetCurrentThreadId();
+#else
+    return (uint64_t) pthread_self();
+#endif
+
+}
+
 } //os
 } //details
 } //spdlog