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/named/config.c

@@ -64,6 +64,9 @@ options {\n\
 #ifndef WIN32
 "	files unlimited;\n"
 #endif
+#ifdef HAVE_GEOIP2
+"	geoip-directory \"" MAXMINDDB_PREFIX "/share/GeoIP2\";\n"
+#endif
 "\
 #	has-old-clients <obsolete>;\n\
 	heartbeat-interval 60;\n\