don't check for stddef.h

don't check for stddef.h

gdb already unconditionally includes stddef.h in many places.
I think there is no reason to check for its existence.

Also, Zack Weinberg's header file survey agrees:

    http://hacks.owlfolio.org/header-survey/

This patch removes the configure check and the inclusion guards.
It also removes a redundant inclusion that I noticed in defs.h.

2013-11-18  Tom Tromey  <tromey@redhat.com>

	* config.in: Rebuild.
	* configure: Rebuild.
	* configure.ac: Don't check for stddef.h.
	* defs.h: Unconditionally include stddef.h.  Remove duplicate
	inclusion.