autoconf-related updates from Ross Younger <ross@crazyscot.com>

autoconf-related updates from Ross Younger <ross@crazyscot.com>

configure.in:
+ require autoconf 2.50
+ use new AC_INIT syntax (NB. This required me to specify an explicit
  package version number and maintainer address!)
+ fix linux/if.h header check
+ add some quoting
+ barf at configure time if gmp, fl or adns are missing
+ use new AC_LIBOBJ syntax for snprintf and getopt
+ include config.h fragments formerly in config.h.bot and config.h.top

Makefile.in: distfiles and dependencies updated to suit.

config.h.bot and config.h.top: removed.