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)

make/rules.in

@@ -118,6 +118,9 @@ LFS_CFLAGS = @LFS_CFLAGS@
 LFS_LDFLAGS = @LFS_LDFLAGS@
 LFS_LIBS = @LFS_LIBS@
 
+MAXMINDDB_CFLAGS = @MAXMINDDB_CFLAGS@
+MAXMINDDB_LIBS = @MAXMINDDB_LIBS@
+
 .SUFFIXES:
 .SUFFIXES: .c .@O@