autoconf: New test for openpty et al.

autoconf: New test for openpty et al.

We may need to #include <libutil.h>, and/or link with -lutil, to use
openpty, login_tty, and the like.  Provide INCLUDE_LIBUTIL_H
(preprocessor constant, not always defined) and PTYFUNCS_LIBS
(makefile variable).

We link libxl against PTYFUNCS_LIBS (which comes from autoconf) rather
than UTIL_LIBS, and #include <libutil.h> where appropriate.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Changes since v7:
 * Actually include the call to AX_CHECK_PTYFUNCS in this patch,
   not the previous one, and regenerate configure accordingly.

Changes since v6:
 * Put failure macro call in correct place so it might actually happen.
 * Try both with -lutil and without.
 * Patch now contains update for config.h.in.
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>