thread.c: suppress warnings only if GCC6+

* thread.c (rb_fd_no_init): suppress warnings by applying ASSUME
  only if GCC 6 or lator, to get rid of freeze with GCC 4.7.2-5.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51894 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -3404,7 +3404,9 @@ rb_fd_select(int n, rb_fdset_t *readfds, rb_fdset_t *writefds, rb_fdset_t *excep
     return select(n, r, w, e, timeout);
 }
 
+#if defined __GNUC__ && __GNUC__ >= 6
 #define rb_fd_no_init(fds) ASSUME(!(fds)->maxfd)
+#endif
 
 #undef FD_ZERO
 #undef FD_SET