Linux-only for now. Some mac bits stubbed out, but not tested.
Good enough for the tiny_race.c example at
https://clang.llvm.org/docs/ThreadSanitizer.html :
$ out/gn/bin/clang -fsanitize=address -g -O1 tiny_race.c $ while true; do ./a.out || echo $? ; done
While here, also make -fsanitize=address work for .c files.
is the lsan stuff needed?