reach_thread_register cannot be compiled on Mac OS as Mac OS
had no pthread barrier.
(the Makefile.am protects this with a if HAVE_PTHREAD_BARRIER)
But the test should then be protected with a prereq that the
executable exists.
(analysis and fix suggestion by rhyskidd, part of fixing 341613)


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14807 a5019735-40e9-0310-863c-91ae7b9d1cf9