The -M switch doesn't cause Build to actually do anything. It causes it to only print the name of the directory in which it will build the program. For sendmail, for example, it would print this:

% ./Build -M
../obj.SunOS.4.1.4.sun4/sendmail

If you have already run Build once to create the object directory, this switch will show the name of that directory. Otherwise, it will print the path to the directory that will be created.