I'm surprised this didn't hit anyone sooner. Doug's portability fix of my

setgid inc autodetection didn't do its caching properly.  It cached the variable
$nmh_cv_ls_mail_grp (previously $nmh_group_mailspool), but did substitution on
$MAIL_SPOOL_GRP (previously $MAILGROUP) without setting the latter to the
former.  That meant that when re-running configure, $MAIL_SPOOL_GRP would be
NULL, so the $(MAIL_SPOOL_GRP) Make variable would be NULL, and installation
would fail with the error "install:  2755 does not exist" (since the argument to
-g was missing).