$sysconfdir is silently overridden with /etc when "--prefix" is not used

@pspacek pointed out to me that when you use ./configure without --prefix, make install puts everything under /usr/local... except the configuration files. I ran ./configure --help, which yielded:

...
Installation directories:
  --prefix=PREFIX         install architecture-independent files in PREFIX
                          [/usr/local]
...
  --sysconfdir=DIR        read-only single-machine data [PREFIX/etc]
...
...

So far so good. Then I checked configure.ac and found this:

#
# Special processing of paths depending on whether --prefix,
# --sysconfdir or --localstatedir arguments were given.  What's
# desired is some compatibility with the way previous versions
# of BIND built; they defaulted to /usr/local for most parts of
# the installation, but named.boot/named.conf was in /etc
# and named.pid was in /var/run.
#
# So ... if none of --prefix, --sysconfdir or --localstatedir are
# specified, set things up that way.  If --prefix is given, use
# it for sysconfdir and localstatedir the way configure normally
# would.  To change the prefix for everything but leave named.conf
# in /etc or named.pid in /var/run, then do this the usual configure way:
# ./configure --prefix=/somewhere --sysconfdir=/etc
# ./configure --prefix=/somewhere --localstatedir=/var
#
# To put named.conf and named.pid in /usr/local with everything else,
# set the prefix explicitly to /usr/local even though that's the default:
# ./configure --prefix=/usr/local
#
case "$prefix" in
	NONE)
		case "$sysconfdir" in
			'${prefix}/etc')
				sysconfdir=/etc
				;;
		esac
		case "$localstatedir" in
			'${prefix}/var')
				localstatedir=/var
				;;
		esac
		;;
esac

git blame indicates that this bit was added in 2000. I think that in 2018 this is wrong and confusing, especially given that ./configure --help claims that the default value for $sysconfdir is PREFIX/etc. IMHO we should at least address this in master.