configure: display summary

This patch adds a message at the end of configure which displays
the different compilation options and system settings.

An example output is the following:

Iptables Configuration:
  IPv4 support:				yes
  IPv6 support:				yes
  Devel support:			yes
  IPQ support:				no
  Large file support:			yes
  BPF utils support:			no

Build parameters:
  Put plugins into executable (static):	no
  Support plugins via dlopen (shared):	yes
  Installation prefix (--prefix):	/usr/local
  Xtables extension directory:		/usr/local/lib/xtables
  Pkg-config directory:			/usr/local/lib/pkgconfig
  Kernel build directory:		/lib/modules/custom
  Host:					x86_64-unknown-linux-gnu
  GCC binary:				gcc

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>