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.

(cherry picked from commit fea6b5bf10)
(cherry picked from commit 6a7e805796)

lib/samples/Makefile.in

@@ -25,7 +25,7 @@ CDEFINES =	@CRYPTO@ -DVERSION=\"${VERSION}\" \
 CWARNINGS =
 
 ISCLIBS =	../isc/libisc.@A@
-DNSLIBS =	../dns/libdns.@A@ @DNS_CRYPTO_LIBS@
+DNSLIBS =	../dns/libdns.@A@ ${MAXMINDDB_LIBS} @DNS_CRYPTO_LIBS@
 ISCCFGLIBS =	../isccfg/libisccfg.@A@
 IRSLIBS =	../irs/libirs.@A@