build: be robust against install-sh files above the root dir

Explicitly set AC_CONFIG_AUX_DIR.

To reproduce the error avoided by this patch:

  rm install-sh        # in case it was already generated.
  touch ../install-sh  # yes, outside this source tree
  autoreconf -fi

This will produce an error like

  ...
  configure.ac:16: error: required file '../ltmain.sh' not found
  configure.ac:5: installing '../missing'
  src/Makefile.am: installing '../depcomp'
  autoreconf: automake failed with exit status: 1

See also automake (vim `which automake`) and look for 'sub locate_aux_dir'.

Change-Id: I3b6d9ad4e5af44c2bdf3844c7bf8e8517bd61d8e

openbsc/configure.ac

@@ -3,6 +3,9 @@ AC_INIT([openbsc],
 	m4_esyscmd([./git-version-gen .tarball-version]),
 	[openbsc@lists.osmocom.org])
 
+dnl *This* is the root dir, even if an install-sh exists in ../ or ../../
+AC_CONFIG_AUX_DIR([.])
+
 AM_INIT_AUTOMAKE([dist-bzip2])
 AC_CONFIG_TESTDIR(tests)