Ordinarily, sendmail's helpfile will be installed automatically. You can see this in part of Build's output:

install -c -o bin -g bin -m 444 helpfile /etc/mail/helpfile

There are legitimate reasons to suppress the installation of this help file. Consider a site that has added legal disclaimers to that file. Such a site might wish to leave the modified file in place, and prevent it from being overwritten during installation of sendmail. To prevent installation of the help file, you can define this confNO_HELPFILE_INSTALL macro:

define(`confNO_HELPFILE_INSTALL')

With this line in your m4 build file, the preceding install line will be eliminated during installation.