build: be robust against install-sh files above the root dir
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: I02153ad52faf1465e9f7821378e04118f17352d2

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts