+When you modify the top-level @file{Makefile.build}, the modified
+version should be automatically copied to
+@file{threads/build/Makefile} when you re-run make. The opposite is
+not true, so any changes will be lost the next time you run @code{make
+clean} from the @file{threads} directory. Therefore, you should
+prefer to edit @file{Makefile.build} (unless your changes are meant to
+be truly temporary).
+