autoconf: trim the configure script; use autoheader

autoconf: trim the configure script; use autoheader

Remove a lot of unnecessary tests.  Specifically, we no longer test
for standard POSIX facilities which we expect to be provided
everywhere and which we don't in any case have any alternative for.

Switch to generating config.h.in with autoheader.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Roger Pau Monne <roger.pau@entel.upc.edu>
Changes since v7:
 * Removed AX_CHECK_PTYFUNCS (snuck in from previous patch)
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>