[Libvir] moving "aux" files into a subdir

[Jim Meyering]

I noticed that gnulib-tool had deposited a file named
link-warning.h into the top-level directory.
How odd, I thought.  This doesn't affect other projects
because most of them define what autoconf calls the "AUX" directory,
with a line like this in configure.in or configure.ac:

AC_CONFIG_AUX_DIR(build-aux)

If you don't define that, it defaults to the top level directory,
and that's where files pulled in by gettextize like these are deposited:

  config.guess
  compile
  libtool
  ltconfig
  ltmain.sh
  config.sub
  config.rpath
  mkinstalldirs

Rather than committing link-warning.h in the top-level, I propose to add
the above AC_CONFIG_AUX_DIR line.  That change should be transparent
to all the other tools, and would let me commit link-warning.h in the
build-aux subdir, instead of in the top-level dir.

Any objection?