fixed compilation error in clang before version 8 (does not support thread_local keyword)

include/spdlog/details/os.h

@@ -315,7 +315,7 @@ inline size_t _thread_id()
 //Return current thread id as size_t (from thread local storage)
 inline size_t thread_id()
 {
-#if defined(_MSC_VER) && (_MSC_VER < 1900)
+#if defined(_MSC_VER) && (_MSC_VER < 1900) || defined(__clang_major__) && (__clang_major__ < 8)
     return _thread_id();
 #else
     static thread_local const size_t tid = _thread_id();