base-gcc doesn't support __thread

Index: thread_pthread.h
--- thread_pthread.h.orig
+++ thread_pthread.h
@@ -72,7 +72,7 @@ typedef struct rb_global_vm_lock_struct {
 
 #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
 #else