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.

bin/plugins/Makefile.in

@@ -17,7 +17,7 @@ CINCLUDES =	-I${srcdir}/include -I${srcdir}/unix/include -I. \
 		${NS_INCLUDES} ${DNS_INCLUDES} \
 		${ISCCFG_INCLUDES} ${ISC_INCLUDES}
 
-DNSLIBS =	../../lib/dns/libdns.@A@ @DNS_CRYPTO_LIBS@
+DNSLIBS =	../../lib/dns/libdns.@A@ ${MAXMINDDB_LIBS} @DNS_CRYPTO_LIBS@
 ISCCFGLIBS =	../../lib/isccfg/libisccfg.@A@
 ISCLIBS =	../../lib/isc/libisc.@A@ ${OPENSSL_LIBS} ${JSON_C_LIBS} ${LIBXML2_LIBS}
 NSLIBS =	../../lib/ns/libns.@A@