build: add autoconf to replace custom checks in tools/check

build: add autoconf to replace custom checks in tools/check

Added autotools magic to replace custom check scripts. The previous
checks have been ported to autoconf, and some additional ones have
been added (plus the suggestions from running autoscan). Two files are
created as a result from executing configure script, config/Tools.mk
and config.h.

conf/Tools.mk is included by tools/Rules.mk, and contains most of the
options previously defined in .config, that can now be set passing
parameters or defining environment variables when executing configure
script.

config.h is only used by libxl/xl to detect yajl_version.h.

[ tools/config.sub and config.guess copied from
  autotools-dev 20100122.1 from Debian squeeze i386,
  which is GPLv2.

  tools/configure generated using the included ./autogen.sh
  which ran autoconf 2.67-2 from Debian squeeze i386.  autoconf
  is GPLv3+ but has a special exception for the autoconf output;
  this exception applies to us and exempts us from complying
  with GPLv3+ for configure, which is good as Xen is GPL2 only.

  - Ian Jackson ]

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Tested-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>