pass --enable-obsolete down to gcc/configure for auto-build.h

Configuring GCC for obsolete targets works as long as build = host.
When it isn't, --enable-obsolete is not passed down to the additional
build configure started by gcc/configure, used to generate
auto-build.h.  The build configure fails and we end up without a
auto-build.h, but the host configure proceeds, so we only get a fatal
failure much later, when make realizes auto-build.h is not there and
there's no rule to create it.

This patch gets the host configure to fail when the build configure
does, leaving the temporary build configure dir behind for
investigation.  It also arranges for --eanble-obsolete to be passed
down to the build configure.

Alas, the latter triggered a warning in the build configure because
--enable-obsolete is not a recognized configure option.  That's not
reported in the host configure because of the
--disable-option-checking passed by the top-level configure, so I
arranged for that to be passed down to the build configure as well.

Finally, since my initial suspicion when investigating this failure
was that auto-build.h had been removed after configuration and there
was no rule to rebuild it, I'm adding rules to gcc/Makefile to get it
created or updated as needed.  Since it is configure that creates it,
as run by e.g. config.status --recheck, and config.status is created
after auto-build.h, I've made config.status depend on auto-build.h,
and added a dummy rule to create auto-build.h.  This would normally
not be enough to create a header when needed, but since Makefile
depends on config.status, and make first updates Makefile, it ends up
working, as long as nothing else that Makefile depends on requires
auto-build.h but not config.status.  The config.status dependency and
the auto-build.h rule are only enabled in the cases in which
auto-build.h is actually used, namely when build != host.


for  gcc/ChangeLog

	* configure.ac: Pass --enable-obsolete=* and
	--enable-option-checking=* down to build configure, and fail
	if it fails.  AC_SUBST HAVE_AUTO_BUILD.
	* configure: Rebuild.
	* Makefile.in [HAVE_AUTO_BUILD] (auto-build.h): New rule.
	[HAVE_AUTO_BUILD] (config.status): Depend on auto-build.h.

From-SVN: r277777