build: set aux-dir

If we don't set AUX_DIR, I get errors during ./configure:
  /bin/sh: /home/david/missing: No such file or directory
  configure: WARNING: 'missing' script is too old or missing

This error is ignored by default, but may cause failures with newer
autoconf versions. Simply set the AUX_DIR to "build-aux" like many other
autotools projects do.

[This is an automake 1.14 bug, see the upstream report:
http://lists.gnu.org/archive/html/bug-automake/2013-11/msg00007.html]

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>