add a search for GeoIP2 libraries in configure

- "--with-geoip" is used to enable the legacy GeoIP library.
- "--with-geoip2" is used to enable the new GeoIP2 library
  (libmaxminddb), and is on by default if the library is found.
- using both "--with-geoip" and "--with-geoip2" at the same time
  is an error.
- an attempt is made to determine the default GeoIP2 database path at
  compile time if pkg-config is able to report the module prefix. if
  this fails, it will be necessary to set the path in named.conf with
  geoip-directory
- Makefiles have been updated, and a stub lib/dns/geoip2.c has been
  added for the eventual GeoIP2 search implementation.

lib/dns/geoip2.c

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+/*! \file */
+
+#include <dns/geoip.h>
+
+#include <isc/util.h>
+
+bool
+dns_geoip_match(const isc_netaddr_t *reqaddr,
+		const dns_geoip_databases_t *geoip,
+		const dns_geoip_elem_t *elt)
+{
+	UNUSED(reqaddr);
+	UNUSED(geoip);
+	UNUSED(elt);
+
+	return (false);
+}
+
+void
+dns_geoip_shutdown(void) {
+	return;
+}