RFA/RFC: Pass --cache-file=/dev/null on to subconfigures

[Ralf Wildenhues]

* DJ Delorie wrote on Fri, Sep 25, 2009 at 07:29:21PM CEST:
> 
> > Well, with upstream Autoconf, you only ever have one cache file anyway,
> 
> Does it properly handle parallel builds simultaneously trying to
> update the cache, yet?

No.  I wasn't aware of this need.  I'll fix Autoconf, and override.m4.

> I've been adding a .NOTPARALLEL: to the toplevel Makefiles of all my
> builds for some time now, to avoid cache collision problems.

So far I had been under the impression that all configure scripts
invoked by the toplevel Makefile would be distinct anyway.  I see now
that that is not the case for subdirs that contain libraries for the
$build system.  I never noticed --enable-serial-configure before; and
there is some more dead code dealing with cache files in toplevel
configure.ac, that should be cleaned up once src has completed the move
to newer Autoconf.  Esp. the host dirs should then be able to once again
share a cache file, I guess.

Thanks,
Ralf