base-gcc doesn't support __thread

Index: thread_pthread.h
--- thread_pthread.h.orig
+++ thread_pthread.h
@@ -93,7 +93,7 @@ struct rb_thread_sched {
 #ifndef RB_THREAD_LOCAL_SPECIFIER_IS_UNSUPPORTED
 # if __STDC_VERSION__ >= 201112
 #   define RB_THREAD_LOCAL_SPECIFIER _Thread_local
-# elif defined(__GNUC__)
+# elif defined(__clang__)
   /* note that ICC (linux) and Clang are covered by __GNUC__ */
 #   define RB_THREAD_LOCAL_SPECIFIER __thread
 # endif