make this link on rh60 - the pthreads that comes with rh60 doesn't allow direct access to __pthread_initial_thread_box, which was only used as a test anyway

gc/boehm/linux_threads.c

@@ -122,8 +122,10 @@ package, we generate a dummy reference to `__pthread_initial_thread_bos',
 which is a symbol defined in LinuxThreads, but (hopefully) not in other
 thread packages.
 */
+#if 0
 extern char * __pthread_initial_thread_bos;
 char **dummy_var_to_force_linux_threads = &__pthread_initial_thread_bos;
+#endif
 
 #define LINUX_THREADS_STACK_SIZE  (2 * 1024 * 1024)