> [makefile] will cause problems if /usr/src is shared between several machines
> with different requirements for which programs are to be installed.

If you really wanted to be twisted, you could symlink "makefile" to something
like "/var/local/makefile.skip.sendmail", which you'd only create on those
machines where you want to suppress sendmail.  (I just checked, make is
perfectly happy to silently ignore a file named "makefile" which is a symlink
to a nonexistant file (as it should).)