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

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).