thread.c: workaround for valgrind on Mac OS X

Without this hack, pthread_join() in rb_thread_stop_timer_thread()
segfaults.

ProductName:	Mac OS X
ProductVersion:	10.11.6
BuildVersion:	15G1510
valgrind-3.12.0

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59053 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread.c

@@ -77,6 +77,9 @@
 #define RUBY_THREAD_PRIORITY_MIN -3
 #endif
 
+#if defined HAVE_VALGRIND_MEMCHECK_H && __APPLE__
+#define THREAD_DEBUG -1
+#endif
 #ifndef THREAD_DEBUG
 #define THREAD_DEBUG 0
 #endif