Use pthread API if the target platform doesn't support TLS attribute

thread_pthread.h

@@ -90,13 +90,25 @@ struct rb_thread_sched {
     int wait_yield;
 };
 
-#if __STDC_VERSION__ >= 201112
-  #define RB_THREAD_LOCAL_SPECIFIER _Thread_local
-#elif defined(__GNUC__) && !defined(RB_THREAD_LOCAL_SPECIFIER_IS_UNSUPPORTED)
+#ifndef RB_THREAD_LOCAL_SPECIFIER_IS_UNSUPPORTED
+# if __STDC_VERSION__ >= 201112
+#   define RB_THREAD_LOCAL_SPECIFIER _Thread_local
+# elif defined(__GNUC__)
   /* note that ICC (linux) and Clang are covered by __GNUC__ */
-  #define RB_THREAD_LOCAL_SPECIFIER __thread
-#else
+#   define RB_THREAD_LOCAL_SPECIFIER __thread
+# endif
+#endif
 
+RUBY_SYMBOL_EXPORT_BEGIN
+#ifdef RB_THREAD_LOCAL_SPECIFIER
+# ifdef __APPLE__
+// on Darwin, TLS can not be accessed across .so
+struct rb_execution_context_struct *rb_current_ec(void);
+void rb_current_ec_set(struct rb_execution_context_struct *);
+# else
+RUBY_EXTERN RB_THREAD_LOCAL_SPECIFIER struct rb_execution_context_struct *ruby_current_ec;
+# endif
+#else
 typedef pthread_key_t native_tls_key_t;
 
 static inline void *
@@ -113,19 +125,8 @@ native_tls_set(native_tls_key_t key, void *ptr)
         rb_bug("pthread_setspecific error");
     }
 }
-#endif
 
-RUBY_SYMBOL_EXPORT_BEGIN
-#ifdef RB_THREAD_LOCAL_SPECIFIER
-  #ifdef __APPLE__
-    // on Darwin, TLS can not be accessed across .so
-    struct rb_execution_context_struct *rb_current_ec(void);
-    void rb_current_ec_set(struct rb_execution_context_struct *);
-  #else
-    RUBY_EXTERN RB_THREAD_LOCAL_SPECIFIER struct rb_execution_context_struct *ruby_current_ec;
-  #endif
-#else
-  RUBY_EXTERN native_tls_key_t ruby_current_ec_key;
+RUBY_EXTERN native_tls_key_t ruby_current_ec_key;
 #endif
 RUBY_SYMBOL_EXPORT_END